On December 19, 1998, the day House Republicans impeached U.S. President Bill Clinton, he recorded his highest-ever score in Gallup’s presidential approval poll: 73 percent favorable.

That was 24 percentage points higher than Clinton’s not-quite-majority 49 percent reelection victory two years earlier. And it was validation of something that by this point had become increasingly obvious during a year of scandal: Clinton had actually received a boost from the impeachment drama, a clarifying event that sharply framed the question, Which side are you on?

Americans looked at the president’s Republican accusers, saw a collection of sanctimonious zealots and power-hungry hypocrites and answered: Not these guys. Clinton boasted afterward that if it were not for the 22nd Amendment, he would have soared to a third term.

President Donald Trump and this generation of Republicans plainly are looking to the politics of the Clinton impeachment as a source of comfort.

They are deluding themselves in a profound way.

Clinton actually received a boost from the impeachment drama, a clarifying event that sharply framed the question, Which side are you on?

Leave aside the argument that many Americans will regard abuse of power in foreign affairs as more serious than lying under oath about extramarital affairs, the reality is that Clinton and Democrats were buoyed by impeachment only in a near-term sense.

Long term, Clinton’s ambitions for his presidency were dealt a grievous blow. And there were enormous, if often overlooked, costs for the progressive movement that are still being paid more than two decades later.

That history echoes with particular relevance today, as public hearings in a new round of impeachment hearings begin. The conventional appraisal of Trump’s prospects — the House will likely convict, the Senate will likely acquit and Trump will claim vindication — might well be true. But this glosses over a larger point: On the current trajectory, Republicans are engaging in a battle with their own long-term costs that they will be paying for the next generation.

Based on Clinton’s precedent, those costs will be paid against Trump’s agenda — things he wants to do but won’t achieve because of the distorting effects of impeachment on his political options and room for maneuver.

They will be paid by his associates — people whose reputations and ambitions will be permanently dented because of their proximity to him.

And they will be paid by conservatives who follow him — who will discover their own principles have lost credibility and power in the public mind because of their connection to Trump.

Understanding the real costs paid by a president during an impeachment battle requires engaging in some what-if scenarios. But these scenarios do not require lots of conjecture or long, speculative leaps.

The frayed center we live with today — with the enormous incentives for politicians to occupy rhetorical and ideological extremes — is not a new phenomenon. As Clinton began his second term in 1997, he believed he was supremely well-positioned to revive and vindicate that center. In an interview with me for the Washington Post a few days before his inauguration, he said his goal was to “flush the poison from the atmosphere” of the capital and of public life more broadly. In his second inaugural address, he quoted Scripture and asked Americans to join him in being “a repairer of the breach.”

On policy matters, Clinton’s notion of the center involved pushing both major parties against old natural instincts. For Democrats, that meant going against the grain on spending and trade, among other issues. For Republicans, it meant if they would surrender their instinctual hostility to government in general, Clinton would work with them in practical ways to create a society in which a robust, technology-driven public sector would work with an efficient future-oriented government to create more opportunities for average Americans.

The frayed center we live with today — with the enormous incentives for politicians to occupy rhetorical and ideological extremes — is not a new phenomenon.

But his primary objective wasn’t narrowly about policy. Clinton believed it was time to end the ideological and cultural wars, heal old racial wounds, and drain public life of its malice and addiction to conspiracy theory. As a newly reelected president and a son of the South, Clinton believed he was uniquely suited to this project.

A year later, these high-minded ambitions collided with the scandal surrounding his relationship with Monica Lewinsky. Once the battle began, Clinton lost all leverage to push his own party — he needed every vote, including those of liberals who had scant interest in his centrist vision — and he had zero pathway to engage even with sympathetic Republicans, whose party leaders brooked no opposition to their plan to evict Clinton from power. The new political and cultural center Clinton tried to create in the 1990s died during impeachment and is still dead.

Clinton wasn’t the only one to pay the price. Vice President Al Gore's obvious resentment of Clinton’s indiscretions and scandal turned him, for a season, into something of a head case — unable to decide how much to embrace Clinton’s economic and policy successes or repudiate his personal behavior. Gore didn’t handle the challenge well, but there’s no question the lingering smoke of impeachment was a genuine obstacle for him in the 2000 campaign. Imagine how different modern history would be if Gore had won the presidency (rather than just the popular vote) that year.

In Hillary Clinton’s case, the costs of impeachment likewise lingered. Even before the year of scandal, she often faced questions about her motives. But for many people, the scandal and the aftermath — staying with her husband and running for Senate — hardened the caricature of her as coldly single-minded in pursuit of power. The scar tissue she accumulated in the 1990s was clearly the inspiration for the private email server that caused such heartache. Imagine how different modern history would be if she had won the presidency (and not just the popular vote) in 2016.

In Hillary Clinton’s case, the costs of impeachment likewise lingered.

What does this history have to do with Trump?

He is used to bullying and insulting fellow Republicans to get his way. Like Clinton, he will suddenly find his ability to push his own party — either on political or policy questions — is deeply limited. Senate Majority Leader Mitch McConnell has already signaled to Trump that he needs to put a sock in it when it comes to criticizing GOP senators.

The reality is that many Washington conservatives dislike Trump as much as Democrats do but are afraid to break with him publicly. These conservatives must ask the cost for their principles. Many conservatives sincerely believe their rhetoric about the need for a strong presidency unshackled from excessive congressional and judicial impediments. People like Dick Cheney and John Yoo face an awkward question: Is this what you had in mind? Empowering a president to shakedown foreign leaders to help his domestic politics or funnel business to his private resorts? The Trump precedent will echo for years in arguments about executive power, and not in ways conservatives will like.

Lastly, most Republicans do not face a high cost within their own party for defending Trump. But, in a country becoming younger and more diverse, there’s little chance even these internal GOP politics remain static. The isolationists of the 1930s had the popular position at the time, but had considerable explaining to do for years after. So did the McCarthy backers of the 1950s. So did the civil rights opponents of the 1960s.

Republican Senator Josh Hawley of Missouri is 39. Republican Senator Tom Cotton of Arkansas is 42. There is every reason to suppose they and other ascendant Republicans will be answering “what did you do in the Trump years?” for decades to come.

As one who covered the Clinton impeachment, I can testify that 21 years is not so long ago as it seems. The smoke from those days passed quickly — the stench lasted far longer.

Altitude is a column by POLITICO founding editor John Harris, offering weekly perspective on politics in a moment of radical disruption.