It’s the 19th anniversary of the House vote to impeach President Bill Clinton. Key Democrats are already planning to spend the 20th on the cusp of doing the same to President Trump.

Democrats must retake the House in the 2018 midterm elections first, which is why they are exercising caution now. There would also likely need to be more significant developments in special counsel Robert Mueller’s Russia investigation than the indictments of Paul Manafort, Rick Gates, George Papadopoulos, or even Michael Flynn, the former national security adviser.

But here are lessons for Trump in how Clinton was acquitted by the Senate and hung on to finish out his term, as well as for congressional Democrats who want to succeed where the 1990s Republican House impeachment managers failed.

The White House and its allies will push to investigate the investigators, accusing them of a partisan “witch hunt” — or worse. That’s the strategy the Clinton White House employed against court-appointed independent counsel Kenneth Starr and his team. Clinton adviser Paul Begala described the Starr investigation as an “ongoing witch hunt” in a 1998 “Meet the Press” appearance, suggesting there also been leaks to reporters. “I think we need a truly independent investigation of the investigation itself so that we can know who's behind these lies, who is behind these leaks, and let the investigation continue,” he said. “I believe that Ken Starr has become corrupt in the sense that Lord Acton meant when he said, 'Absolute power corrupts absolutely.' ''

"This is rough business and Starr plays it rough,'' an unnamed Clinton adviser told the New York Times around the same time. ''You can say a lot of things about us, but we're not patsies. The President is not a quitter and the folks around him are fighters. We think the President has been treated poorly and unfairly by Starr and we're not going to take it.''

“The Paula Jones and Whitewater investigations existed only because of the efforts of Clinton’s right-wing political enemies,” the liberal columnist Anthony Lewis wrote in a subsequent review of a book by staunch Clinton apologists Joe Conason and Gene Lyons, along with a second related volume by Jeffrey Toobin. “People who hated the Clintons initiated these projects and sustained them through many years.” The New York Review of Books titled Lewis’ piece “Nearly A Coup.”

It was easier to paint Starr as a right-winger than Mueller — a Republican who has served under presidents of both parties — as a left-winger, though the Trump-Russia special counsel is close to James Comey, a fellow former FBI director, whose firing by Trump has emerged as a central question in the probe. But there are other members of the Mueller team against whom a case for political bias can be more readily made and the FBI's Peter Strzok has already been tossed from the probe over pro-Hillary Clinton, anti-Trump texts, including one making a cryptic reference to some agency course of action being like an “insurance policy” if Trump was elected.

“Now it’s time for subpoenas,” Rep. Jim Jordan, R-Ohio, a top Freedom Caucus member, told Fox Business on Tuesday. “And their chairman of the Judiciary Committee is going to subpoena Lisa Page, he’s going to subpoena Bruce Ohr, and he’s going to subpoena Peter Strzok, and we’re also going to get eventually to [deputy FBI director] Andrew McCabe.”

“We are at risk of a coup d'etat in this country if we allow an unaccountable person, with no oversight, to undermine the duly-elected President of the United States,” said Rep. Matt Gaetz, R-Fla., on the House floor. A look back at the Clinton impeachment saga shows this is not unprecedented talk.

Partisan impeachments are tough. Only five House Democrats voted for any of the articles of impeachment drafted against Clinton nearly two decades ago, including just one with a mostly liberal voting record. Even before the Senate trial, zero Democratic senators were willing to vote to convict Clinton, making his removal from office impossible. Ten Republicans actually voted “not guilty” on the charge of perjury against the 42nd president, five voted to acquit him on the charge of obstruction of justice. A two-thirds majority was needed to end Clinton’s presidency; the country had no consensus in favor of a such a move at the time.

Ten Democratic senators represent states Trump carried in 2016. Sen. Joe Manchin, D-WV, serves a state that went for the president by 42 points. Even if they all voted to convict Trump, under the chamber’s current composition they would need at least 16 Republicans to join them. Let’s say they not only win the House next year but also take over the Senate, despite a GOP-friendly map. In a 55-45 Democratic Senate — a wildly optimistic scenario — 12 Republicans would have to vote to convict Trump, many of them in states where he could still be popular at the time.

Clinton was actually able to use impeachment to rally the country behind him, which seems unlikely under Trump (though anything is possible if the economic recovery keeps humming along). On the other hand, this fight has the potential to become even more partisan, partly because Republicans are still running against [Hillary] Clinton’s parallel set of scandals, which they contend were not taken as seriously by the FBI. “We’ve got to clean this town up, and it will start with the resignation of Mr. Mueller and a proper investigation of all of this underlying case involving Comey, [Loretta] Lynch, the Clintons, and Russia,” said Rep. Louie Gohmert, R-Texas.

Trump has criticized his own Justice Department for slow-walking Hillary-related investigations. He has two big disadvantages compared to Clinton. His attacks on Mueller will be far less favorably covered by the press than Clinton’s condemnation of Starr. Clinton launched his counterstrikes with approval ratings in excess of 60 percent and only a third of the country supporting impeachment. Trump’s approval rating hovers between 35 and 40 percent and some polls have found significant support.

Still, Democratic leaders have been trying to keep rank-and-file members who are eager to impeach Trump from getting too far ahead of themselves.

Sex wasn’t considered serious enough to impeach a president over, but Russia might be. After a lengthy investigation of Whitewater and other matters by more than one independent counsel, Clinton was ultimately impeached for perjury and obstruction of justice concerning his testimony about White House intern Monica Lewinsky during the Paula Jones sexual harassment suit. So far the investigation into whether Trump associates colluded with Russian interference in the 2016 presidential election has yielded indictments related to financial crimes that predate the campaign and process crimes from the campaign, with none of the charges asserting the existence of a broader conspiracy.

Team Trump no doubt hopes it stays that way, though it is still early. But Clinton was impeached by the House for obstruction of justice, as well as perjury. He was not convicted in part because no Senate Democrats believed the underlying crimes were serious enough to remove him from office — he was, they often argued at the time, only lying about sex.

In the post-Harvey Weinstein climate, it’s possible that even lying about sex might be considered more impeachable today, at least in the context Clinton was asked about it. If Mueller builds an obstruction case against Trump, the underlying issue will be interference in an investigation about malign Russian influence in an American election. Depending on how strong Mueller’s evidence is, it might be considered more publicly relevant — and a more appropriate subject for a serious Senate trial.

The outcome of the Mueller investigation, much less an anti-Trump impeachment push, is still undetermined. A lot of the script leading up to it, however, was written almost two decades ago.