(CNN) History signals that the public's final verdict on President Donald Trump's possible impeachment won't be delivered until the 2020 election -- whatever happens next in the House and Senate, and however Americans react to it.

The public reactions to impeachment proceedings against Presidents Richard Nixon in 1974 and Bill Clinton in 1998 fundamentally diverged , with most Americans ultimately supporting the former and a consistent majority opposing the latter.

Yet the outcomes in the next presidential elections converged. In each case, the president's party lost the White House to a candidate -- Democrat Jimmy Carter in 1976 and Republican George W. Bush in 2000 -- who played off lingering public unease about the scandal that had precipitated the impeachment process against their predecessor. In each instance, impeachment functioned like a leak that corroded the foundation under the president's party in the next election.

Those precedents suggest the revelations about Trump's conduct in the Ukraine investigation could complicate his path to a second term, whatever polls say now about the public response to his potential impeachment and removal from office. Recent polls indicate that while the controversy is not reducing the share of Americans who approve of Trump's job performance, it does appear to be hardening the opposition he faces among those who disapprove of him.

"The whole impeachment debate has intensified preexisting feelings about the President," says Republican pollster Whit Ayres. "Most people who like him do so with their eyes wide open. ... They understand his strengths and his weaknesses and they prefer him to the alternatives as they see it. On the other hand, people who dislike him will be even more appalled because what they are seeing reinforces their existing views."

The risk for Trump in that dynamic is that a majority of 51 to 55% of Americans have consistently said they disapprove of his performance, while only around 40 to 45% have usually said they approve. Even if Americans remain closely split on whether Congress should impeach and remove Trump from office, the scandal may be solidifying a division of attitudes that forces him to find a path to reelection without support from anything close to a majority of the electorate.

"At the very least it is deepening a lot of Trump's problems," says Democratic pollster Geoff Garin. "I think it is confirming for voters that Trump is a person lacking in integrity and ethical standards in a way that is particularly concerning to college-educated men. And I don't think we are done with the impeachment story yet."

Striking similarities in campaigns

Impeachment provided a critical backdrop to both the 1976 and 2000 elections, which followed the proceedings against Nixon and Clinton. Nixon resigned in August 1974 after the House Judiciary Committee approved impeachment articles against him; the Senate failed to remove Clinton in February 1999, after the Republican-controlled House of Representatives voted on a near-party-line basis to impeach him in December 1998.

The public reaction to these two confrontations utterly diverged. During the long gestation of the Watergate scandal, from early 1973 through Nixon's resignation, support for his impeachment slowly grew and his job approval rating steadily eroded. In Gallup polling, a majority of Americans supported Nixon's removal from office only in the final survey before his resignation; by then his approval rating had fallen to about 24%, though it remained around 50% among Republicans.

By comparison, public opinion remained remarkably stable through 1998 during the House GOP's long march toward impeaching Clinton. Consistently in Gallup polling through that year, Americans opposed his impeachment and removal by about 2 to 1, and his approval rating remained well above 60%.

Yet, despite these differences in the immediate public reaction, the presidential campaigns that followed these two impeachment proceedings unfolded with some striking parallels. In each case, the party that drove the impeachment inquiry won the next presidential election. And in each instance, that party did so behind a nominee who did not dwell on the scandal that had prompted impeachment -- but unmistakably presented himself as the antidote to it.

Jimmy Carter, the Democratic former Georgia governor, did not stress Watergate during the 1976 campaign, insisting the American people had heard enough about it. But Watergate still infused everything about his campaign. Carter centered his candidacy not on a big, ambitious policy agenda but on an overriding promise to unify the nation and restore to America "a government as good as its people." That struck a powerful chord with voters exhausted by Watergate and allowed Carter, a one-term Georgia governor and former peanut farmer, to overcome doubts about his experience and narrowly defeat Gerald Ford, who had served as Nixon's vice president and then succeeded him as President after Nixon's resignation.

In 2000, George W. Bush likewise did not dwell on the affair with a White House intern that triggered Clinton's impeachment. But it too infused Bush's campaign. Like Carter, Bush presented himself as a unifying figure, "a uniter, not a divider," in his phrase. And the Clinton scandal provided the backdrop for Bush's core promise in 2000: to "restore honor and dignity to the Oval Office." That messaging helped Bush squeeze out his narrow Electoral College victory over Al Gore, Clinton's vice president, even while narrowly losing the popular vote.

As Tad Devine, one of Gore's top strategists, told me earlier this year , Bush skillfully found ways to tap the unease over Clinton's behavior without directly embracing the unpopular impeachment itself.

"It became a very valuable tool even though Bush didn't go around saying, 'Impeachment, impeachment,' " Devine told me. "He took the bad stuff from impeachment and put it front and center [in the campaign]. What could Gore say? 'I'll restore honor too'?"

Trump's situation is different

Public opinion over Trump's possible impeachment has followed a middle course between these two examples. Support for impeachment jumped to around 50% after Democrats formally announced their inquiry in September but has stagnated since then. Trump's job approval ratings have remained almost unchanged: In a new CNN survey conducted by SSRS and released this week, 50% of Americans supported Trump's impeachment and removal, a number unchanged since October. His job approval rating stood at just 42%, with 54% disapproving. Through all of this year, his approval rating in the CNN poll has varied only between 39% and 43%.

In another important regard, Trump's situation differs decisively from these precedents: Unlike Nixon and Clinton, Trump himself will be on the ballot in 2020 if the House impeaches him and the Senate does not vote to remove him from office. Under that scenario, Trump would become the first impeached president to appear on the next general election ballot. (The only other president who faced this sanction was Andrew Johnson , Abraham Lincoln's vice president, who was impeached but not removed by the Republican-led Congress in 1868; Johnson sought to run again that year as a Democrat but didn't win the nomination.)

Trump's presence on the ballot might change the dynamic from the Nixon and Clinton precedents in some respects -- but likely won't alter them in others. The biggest difference is that unlike Ford or Gore -- who did not attempt to relitigate whether impeachment had been appropriate for the presidents they had served under -- Trump will likely try to mobilize more turnout from his core backers by insisting he was unfairly persecuted by an "unholy alliance" of Democrats, the media and the deep state, as he put it during a recent rally

Ayres, the Republican pollster, says Trump's unbending response to impeachment underlines the pugnacious take-no-prisoners posture that thrills his core supporters.

"Supporters take away he's a fighter, that he's never backs down, that he never gives in to the carping critics and that he will stand his ground no matter what the situation and hold his head high in the face of unrelenting criticism," he said.

But Ayres, like a wide array of Democratic strategists, agrees the revelations in the Ukraine investigation are serving just as powerfully to reinforce the doubts Trump faces among those skeptical of him.

"It is another strike against him," says Josh Schwerin, senior strategist and communications director for Priorities USA, a leading Democratic Super PAC. "It reinforces the negative feelings they have about his actions and the way he carries himself."

Trump as lesser of two evils?

That dynamic is evident in recent poll results showing that overwhelming majorities of voters who disapprove of Trump's overall job performance view his actions in Ukraine as deeply troubling.

In one recent Quinnipiac University national poll 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. Another Quinnipiac Poll found that 94% of those who disapprove of Trump's job performance say he was pursuing his own interests, and not the nation's, in Ukraine.

In the new CNN survey, 90% of those who disapprove of Trump overall say that in his dealings with Ukraine he abused his presidential power to gain political advantage. Likewise, in the latest ABC/Washington Post survey, 88% of disapprovers said he did something wrong in his dealings with Ukraine, according to figures provided by the Post. Fully 89% of those who disapproved of Trump in the CNN survey considered his behavior so egregious that they endorsed the unprecedented step of removing him from office.

Whether or not most Americans support removing Trump from office before his term, strategists in both parties generally agree that such judgments could complicate a critical part of the GOP plan for 2020. One of the Trump campaign's hopes is that it can convince some voters who now say they disapprove of him to vote for him nonetheless by making the Democratic nominee more unacceptable to them. To the extent the Ukraine investigation deepens the concerns of those disapproving voters, convincing them to accept Trump as the lesser of two evils obviously becomes more difficult. Schwerin says the key issue isn't whether voters who disapprove of Trump believe he should be removed now, but whether they can accept more of the behavior unearthed in the investigation for another four years.

One of the most intriguing questions for 2020 is whether these dynamics will create any fissures among the different factions in the Republican coalition.

In polling, all elements of the Republican coalition have unified against the Democratic-led impeachment (with the exception of the recent ABC/Washington Post poll that found that just over one-sixth of Republicans believe Trump should be removed.) But polling hints that after the threat of Trump's removal is gone --either because the House fails to impeach or the Senate refuses to convict --Republicans may differ more over the underlying behavior that the impeachment inquiry has revealed.

The recent annual national American Values Survey by the nonpartisan Public Religion Research Institute divided Republicans between the roughly 45% who said they receive their information mostly from Fox News Channel and the 55% who said they rely mostly on other sources. On impeachment these two Republican groups differed only modestly, with the idea generating opposition from 98% of the Fox watchers and 90% of the others.

But on other relevant measures, the groups did split more. While 71% of the Fox watchers said they strongly approved of Trump's performance in office, only 39% of the non-Fox watchers agreed; roughly one-fifth of the latter group said they disapproved of him. One-third of those who relied on other sources said Trump has damaged the dignity of the presidency -- compared with just 1 in 11 of the Fox watchers). In the CNN survey, almost 1 in 6 Republicans and independents who lean toward the party said Trump abused his powers in Ukraine.

Will weariness grow into opposition?

Stanley B. Greenberg, the veteran Democratic pollster, sees openings for Democrats in those cracks. In a recent round of focus groups around the country, he said he found that some Republicans, even if they support Trump's policy goals, are growing exhausted by the relentless controversy and division surrounding him. The Ukraine scandal and impeachment fight, Greenberg believes, could crystallize those concerns. Greenberg says "probably the most important impact" of the ongoing impeachment struggle is that "it only reinforces that we'll have continued polarization and divisiveness" if Trump is reelected. "It doesn't create hope" for something different, Greenberg says.

Given the underlying stability in public opinion about Trump, it appears unlikely that Democrats can amass a big majority to support his impeachment (absent perhaps some bombshell revelation from one of his inner circle who has so far refused to testify). And another few months with the impeachment debate dominating national news coverage -- first in the House and then possibly in the Senate -- will allow Republicans to raise charges of corruption in Ukraine against former Vice President Joe Biden and his son Hunter Biden, even though those allegations have been widely discredited by the media and in the House testimony from current and former Trump administration officials. Weeks more of impeachment debate in the House and an impeachment trial in the Senate could also help Trump energize his base of supporters for 2020.

But more attention to Trump's behavior in Ukraine may also provide a stronger opportunity for the next Democratic nominee to reprise the core strategy of the winning candidates after the past two impeachments. Just as Carter in 1976 promised honesty and Bush in 2000 pledged decency, the next Democratic nominee may promise a return to normalcy after a Trump term defined by unending volatility, confrontation and collisions with legal boundaries.

Biden previewed such arguments last weekend in Iowa; in a distant echo of Bush in 2000, he declared that one of the first responsibilities for the next president will be to "restore dignity to the White House."

However much impeachment allows Trump to mobilize his base, the confrontation may strike many other voters as an uncomfortable preview of a second Trump term that extends -- and likely even enlarges -- the unstinting controversies of his first.

"There are a lot of negatives on Trump," says John Anzalone, a Democratic pollster advising Biden's campaign. "But at the end of the day I think people are so exhausted by him and they so want their lives back that that will be a really big psychological driving factor."