Once Democratic control of the House of Representatives made the impeachment of Donald Trump hypothetically possible, discussion of the topic has been accompanied by a widespread assumption (held inside as well as outside the party) that pursuing impeachment is a high-risk political strategy with a very good chance of seriously backfiring regardless of its substantive justification. This conventional wisdom reflects the Washington community's collective memory of the events of 1998, which goes something like this: zealous Republicans insisted on impeaching Bill Clinton despite strong opposition from American voters, who exacted their revenge by raining electoral blows upon the GOP in the congressional midterms as Clinton snickered with delight.
But that's not really what happened. It's true that impeachment was unpopular, but the actual results of the 1998 election were hardly calamitous for Republicans. The party suffered a very minor loss of 5 seats in the House, while there was no net partisan change in the Senate. Republicans held a majority in both congressional chambers prior to the election, and they retained this majority afterwards with little change in their margin of control. The 1998 midterms were the epitome of a status quo election, with no measurable wind or wave in either direction.
There are two main reasons why the Clinton impeachment is misleadingly remembered as such a disaster for the Republican Party. The first is that most journalists and commentators assumed that Republicans would reap major political benefits from the Clinton-Lewinsky scandal from the moment it first emerged in January 1998. Even when polls began to roll in showing that most Americans weren't outraged by the revelations—and certainly didn't believe they warranted removal from office—pundits repeatedly waved them off, insisting that the electorate would be properly scandalized once it learned more of the facts. For example, Sam Donaldson of ABC News was predicting as late as mid-October that the public was about to turn against Clinton for good, forcing his resignation from the presidency.
The written report of special prosecutor Kenneth Starr, engineered by congressional Republicans to be released to the public in the midst of the campaign season in mid-September, contained a number of salacious details that consumed the news media for weeks. When combined with the "iron law" that the president's party "always" loses House seats in the midterm election (which had held true for every midterm since 1938 at that point), this well-timed additional exposé was supposedly poised to hand the GOP a major political advantage. So while the election itself hardly produced a Democratic landslide, the outcome was interpreted against a backdrop of contrary expectations as a decisive victory for Clinton at the expense of his partisan opponents.
The second reason why the Clinton impeachment went down in history as a political fiasco is the messy leadership succession that occurred after the election. House Speaker Newt Gingrich was immediately forced out by his own party, though the disappointing election returns represented the final blow to an already weakened Gingrich (who had faced an internal Republican coup attempt the year before) rather than the central factor precipitating his exit from the speakership. It was easy enough for media reports to frame Gingrich's downfall as representing Clinton's ultimate triumph over an arch-nemesis, but the abrupt withdrawal of the next prospective speaker, Bob Livingston, from consideration for the job several weeks later—Livingston had carried on an extramarital affair, which became disqualifying under the circumstances on grounds of perceived partisan hypocrisy—merely cemented perceptions that the whole production had turned into an utter wipeout for Republicans even before the Senate trial of Clinton ended in an anti-climactic acquittal early the following year.
But there is no clear evidence that Republican candidates as a group performed any worse, either in 1998 or thereafter, than they would have absent the impeachment push. Clinton was an unusually popular president due primarily to an unusually robust national economy, and many of the most promising congressional seat targets for the GOP had already been picked off in the 1994 or 1996 elections. Congressional elections expert Gary Jacobson of UC San Diego concluded at the time that "the results of the 1998 elections are in no way extraordinary. . . . [they] are about what we would expect if no one had ever heard of Monica Lewinsky."
If Clinton's impeachment didn't really matter much in the 1998 elections (Washington lore aside), there's little reason to believe that a potential impeachment of Donald Trump will be decisive in 2020. Trump is much less popular than Clinton was; public attitudes have become more consistently partisan and less malleable over the 21 succeeding years; fewer members of Congress are cross-pressured by representing a constituency that normally leans toward the opposite party; and even an impeachment process that stretches on for a few months will conclude well before next November. The currently developing scandal is important for a number of reasons, but it's very possible that its influence on future elections never extends beyond potentially pushing this or that odd congressional seat in one partisan direction or the other.
An impeachment proceeding is such a momentous, historic event that it's only natural to expect it to have a transformative effect on the attitudes of the American public. This was certainly a common assumption in 1998—how can such a big political story not produce a correspondingly big electoral impact?—and some analysts continue even today to treat impeachment as a perilous political proposition for one side or the other. But the Trump presidency has already generated more than the usual number of big political stories, and the effects of all of them on mass opinion have been modest at best. Americans have already made up their minds about this president, and it will take a truly dramatic set of future developments for most of them to re-evaluate his performance in office. That doesn't mean this isn't an important story—it surely is, just as Clinton's impeachment was. But not everything that's important in politics leaves a major imprint on the voting returns.