That unique prospect could scramble the electoral calculations next year for both parties.
As impeachment proceeds, the division in the country "is going to go into the stratosphere," predicts Charles Coughlin, a veteran Republican political strategist based in Phoenix. "Which I think creates an opportunity for a candidate ... to fill that narrative: We have to start talking about what brings us together and not what pushes us apart. I think there will be giant pieces of room in the electorate, both Republican and Democratic, to articulate that notion."
One thing that's clear already is that if the House votes to impeach Trump - and the Senate does not reach the two-thirds majority required to remove him from office -- the nation will face a novel political situation in 2020. None of the previous three presidents who faced a serious impeachment threat appeared on the next general election ballot.
Richard Nixon retreated into exile in San Clemente and never again sought public office after he resigned in August 1974. His resignation came after the House Judiciary Committee approved articles of impeachment against him over Watergate with bipartisan support, and leading Senate Republicans told him his support in that chamber was crumbling.
Bill Clinton finished his second term as President but was barred by the Constitution's two-term limit from running again in 2000. The Republican-controlled House voted to impeach him in 1998 but the Senate fell well short of the votes needed to remove him over the perjury charges that emerged from his affair with a White House intern.
Only Andrew Johnson, among the presidents facing impeachment, sought to run again after his ordeal. Abraham Lincoln, the first Republican president, picked Johnson, a pro-Union Democrat from the border state of Tennessee, as his vice president in 1864 to create a national "unity" ticket. But after Lincoln was assassinated in 1865 and Johnson ascended to the presidency, he reverted to his roots in a Democratic Party that, at the time, was much more sympathetic to the South and hostile to the rights of freed slaves. Johnson repeatedly tried to stymie the agenda of the Republican House and Senate majorities to ensure rights for the freed slaves and eliminate Confederate influence in the Southern states rejoining the Union.
After an escalating series of confrontations and vetoes, the House in March 1868 impeached Johnson on a party-line vote; all Republicans who voted backed impeachment and all voting Democrats opposed it. The Senate then fell one vote short of removing Johnson from office in May, just months before the 1868 election. All Senate Democrats and seven Republicans voted to acquit him on the impeachment charges, which centered on his defiance of a law the Republican majorities passed to prevent him from dismissing executive branch officials without congressional approval.
Johnson's strategic move
David O. Stewart, author of the 2009 book "Impeached: The Trial of President Andrew Johnson and the Fight for Lincoln's Legacy," said the nearness of the next election was one reason that just enough senators declined to impeach Johnson.
"I actually think the impending election probably helped him resist the impeachment case because there were a bunch of people who said, 'He is going to be gone pretty soon so why are we killing ourselves?' " Stewart said in an interview. "And there was a fear of a backlash that they would make him sympathetic by pursuing him."
Having survived the threat of removal by his adopted party in Congress, Johnson then came full circle in his partisan migration: He sought to win the Democratic nomination in 1868 to face the formidable Republican choice, Ulysses S. Grant, the general of the North's victorious armies during the war.
"Johnson desperately wanted to be nominated, and it was for vindication and he thought he could win," said Stewart. "He thought Grant was vulnerable."
Johnson sent intermediaries to make his case at the Democratic convention in New York. On the first ballot, he finished second, but it was quickly clear those votes were more a gesture of respect than a reflection of any genuine desire to choose him. Johnson quickly faded from contention and Democrats on their 22nd ballot chose as their nominee New York Gov. Horatio Seymour, who was promptly beaten by Grant.
Because there are so few precedents for how an impeachment may affect the next election - and no precedent for the impeached president appearing on the ballot again -- experts are cautious about offering too many predictions of how today's confrontation could affect 2020.
"Anybody who says they know what is going to happen is wrong," says Tom Rath, a longtime Republican operative in New Hampshire. "There is no easy parallel to find."
A few common threads
But some common threads do run through these previous struggles, even if no one can say for sure how relevant they will prove in 2020.
One is that in each previous case, the party that drove the impeachment won the White House in the next election: Republicans triumphed in 1868 and 2000, and Democrats won in 1976. Despite predictions of a backlash, in each case the country turned away from the president's party in the succeeding election.
Another is that in each case, the winning candidate promised to restore national unity after the bitter partisan conflict.
Grant, as the leader of the victorious army during the Civil War, was a living symbol of Union. And though he was clearly identified as a champion of the North, Grant, to the extent candidates made their views known in those years, also signaled his determination to pursue reconciliation with the South. In his inaugural address, he declared that all issues relating to Reconstruction must "be approached calmly, without prejudice, hate or sectional pride." (Brutal violence by Southerners opposed to rights for the freed slaves forced Grant to intervene in the former Confederate states more aggressively than those words implied.)
After the "long national nightmare" of Watergate and Nixon's resignation, Jimmy Carter in 1976 also won election after presenting himself as a unifying figure who would restore to America "a government as good as its people." As an evangelical Christian who quoted Bob Dylan in his acceptance speech at the 1976 Democratic Convention, a Southerner who supported civil rights and a centrist governor who blended liberal and conservative themes, Carter embodied the promise of cultural, racial and political reconciliation after not only the bruising Nixon years but also the larger tumults of the 1960s.
"Our country's lived through a time of torment," Carter declared in that acceptance speech. "It's now a time for healing. We want to have faith again. We want to be proud again. We just want the truth again."
Trump likely to take a different tack
In 2000, after Clinton's impeachment, Republican George W. Bush won by presenting himself as a "uniter, not a divider" who had worked closely with Democrats in the Texas state Legislature. Bush criticized Clinton's behavior but only indirectly, through his pledge to "restore honor and dignity to the Oval Office," a cornerstone of his campaign.
And Bush distanced himself at times from congressional Republicans, pledged to seek common cause with the centrist "new Democrat" movement that Clinton led and lamented what he called "a cycle of bitterness, an arms race of anger."
Though partisan conflict grew over his two terms, especially after the Iraq War, Bush initially ran in 2000 on a message of inclusion: "I will bring Americans together," he said.
No one expects Trump to follow those tracks, if he remains in office after the impeachment inquiry and is able to seek reelection, as now seems most likely.
Trump, to a unique extent among presidents, has focused far more on energizing and outraging his base than on trying to expand it. Continuing that approach, he has responded to the risk of impeachment with an unprecedented torrent of accusations against his critics, accusing House Democrats of a "coup," "treason" and "a brazen attempt to overthrow our government" and retweeting a prediction that a vote to remove him could open a "civil war-like fracture" in the country.
If anything, the impeachment struggle may push Trump further toward a strategy of energizing his core supporters and turning out more of them who did not vote in 2016. The reason is that the process could complicate his hopes of capturing voters ambivalent or conflicted about his presidency.
Bad impressions solidifying
So far there's not clear evidence in polling that the allegations against Trump related to Ukraine
, or his belligerent response to them, are increasing the number of voters who disapprove of his performance in office or reducing the number who approve.
But recent polling suggests that the new controversy is hardening the most negative impressions of Trump among the majority of voters who have consistently disapproved of him during his presidency.
"For people who have this uneasiness about him, I think this is just a really different event" from the Russia investigation, says Democratic pollster Anna Greenberg. Unlike that controversy, she says, "What happened in Ukraine is not about the past; it's about the future. ... And it's just letting the things about him that people don't like come to the fore in this dramatic way."
In an NPR/PBS NewsHour/Marist Poll released last week, for instance, fully 93% of voters who said they disapprove of Trump's overall performance in office said it was not acceptable "for a president to ask a foreign country´s leader to help investigate a potential political opponent," according to detailed figures provided to me by Marist. Among those disapproving voters, 92% also said Trump was mostly serving his campaign's interests, not the country's, in his call with Ukrainian President Volodymyr Zelensky.
Similarly, a recent Quinnipiac University national poll found that among voters who disapprove of Trump's job performance, 94% said he has abused the power of his office and 93% said he considers himself above the law, according to figures provided by the pollsters.
These sentiments pose a clear challenge to a cornerstone of the Republican strategy for 2020. Republicans believe Trump can survive majority disapproval of his performance by disqualifying the eventual Democratic nominee to the point that some voters who disapprove of Trump vote for him anyway. No one in either party rules out Trump's capacity to damage his opponent. But to the extent the impeachment fight reinforces negative views about Trump -- such as the belief that he considers himself above the law or puts his personal interests above the nation's -- winning voters who disapprove of his performance will only grow more difficult for him, whatever their views about the Democratic alternative.
"It is getting harder and harder for him to convince people who don't support his performance (in office) that they should support him for reelection," says Josh Schwerin, a senior strategist for Priorities USA, a leading Democratic super PAC. "There is no good scenario here for Trump, absent some exonerating fact that doesn't seem possible. Given the facts and the numbers we are seeing, it is hard to believe he doesn't get impeached by the House and no matter what happens in the Senate it is going to be a drag on him."
Democratic primary could also be affected
Indeed, in last week's Marist Poll, fully 95% of voters who disapproved of Trump's job performance also said they now intend to vote against him for reelection. Eight-five percent of those who disapproved in the survey endorsed the extraordinary step of the Senate removing him from office.
One of the biggest unanswered questions impeachment raises is whether the confrontation will change the kind of alternative that most voters skeptical of Trump will find appealing in 2020.
The dominant assumption among the Democratic presidential contenders gathering for a CNN debate tonight in Ohio is that after four years of searing partisan combat with Trump and congressional Republicans, the party wants a "fighter"; even former Vice President Joe Biden
, who has offered a more centrist message than his principal rivals, is promising to "beat (Trump) like a drum."
The Carter and Bush precedents, though, suggest there may be more of an audience than Democrats now expect for a conciliatory candidate in 2020. An impeachment fight that further fans the flames of partisan conflict could make voters less enthusiastic about replacing a conservative President who revels in political combat with a liberal, such as Sens. Bernie Sanders of Vermont or Elizabeth Warren of Massachusetts, who promises unending confrontation over a different set of issues.
One final twist from the Johnson impeachment may unsettle both parties. As soon as Johnson left the White House, as Stewart recounts, he began maneuvering to return to Congress to continue his campaign against the postwar reconstruction of the South. In 1869, a time when US senators were picked by state legislatures, Johnson campaigned for an appointment in Tennessee, but fell just short. In 1872, he ran unsuccessfully for the US House. Finally, in 1875 he achieved a measure of vindication when the Tennessee state Legislature finally appointed him to a Senate seat. The satisfaction was fleeting: About four months after Johnson returned to Washington, his resentments still burning, he died from a stroke.
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