Some have embraced the effort, while others have sidestepped it, wary of turning the November elections into a referendum on impeachment — that Republicans intend to portray Democrats as unhinged by the president, and out of ideas on jobs or health care.
Perhaps heightening that peril is the ascension of Rep. Jerrold Nadler (D-N.Y.), a constitutional law expert with firsthand experience battling Trump, to the position of ranking Democrat of the House Judiciary Committee. It is the surest sign yet of how seriously House Democrats are preparing for a legal showdown with the president.
In one year, if Democrats win control of the House of Representatives, Nadler will become Judiciary chairman. At a recent town hall in his Upper West Side district, Nadler's constituents made clear what they want him to do.
One constituent asked about the American Civil Liberties Union's lawsuits against the White House; another about the Department of Justice rescinding a rule that allowed states to legalize marijuana. Another asked if House Republicans were engaging in obstruction of justice by attempting to thwart investigations into whether the Trump campaign worked with Russians to meddle in the 2016 election.
"Congress is not exercising its oversight," Nadler warned. "You don't do certain things. You don't put your own personal financial benefit in the middle of your government. You don't take gifts from foreign governments."
He said later: "There's so much to look into, but we can't call a hearing."
Republicans want voters to know that — and to be wary. Nervous about their control of Congress, they intend to portray Democrats as unhinged by Trump, unable to work with him and bent on impeaching him.
"It's a great contrast," said Corry Bliss, the executive director of the Congressional Leadership Fund, a super PAC aligned with House Speaker Paul D. Ryan (R-Wis.). "One party is cutting middle-class taxes. The other party's spending its time figuring out how to impeach the president."
That messaging scares some Democrats. To win the House, they need to win 24 seats — most of them in districts Trump lost, some in districts that Republicans usually win comfortably. Polling on Trump's fitness for office, or on ways Congress can remove him, has shown more support for drastic measures than at any time since the Watergate hearings. In November, a Quinnipiac poll found 54 percent of all voters — and 91 percent of Democrats — say that Trump was simply unfit for office.
It's still not an argument some Democrats want to have. Some have bristled at the "Need to Impeach" ad campaign by Tom Steyer, the billionaire Democratic donor, worrying that it will let Republicans put their candidates on the spot. Most have opposed Rep. Al Green (D-Tex.), who is not a member of the Judiciary Committee, in his well-publicized efforts to bring an impeachment resolution to the House floor.
Rep. Matthew Cartwright (D-Pa.), who is running for reelection in a district where Trump defeated Clinton by about 10 points, has distanced himself from the impeachment talk.
"I think there are two levels," Cartwright said in an interview. "There's the legal analysis and then there's the political one. Under both analyses, impeachment is not appropriate."
The tension was also evident in Phoenix over the weekend, where RNC committeeman Bruce Ash warned an audience of Arizona Republicans that a Democratic victory would quickly lead to impeachment.
"Friends, if they win back the House, count on it: Within six months, there'll be an impeachment resolution," Ash said. "We can't let that happen."
An hour earlier, Rep. Ruben Gallego (D-Ariz.) told an audience outside a doughnut shop that he was not ready to talk impeachment but wanted special counsel Robert S. Mueller III first to finish his work on the Russia investigation.
"I've been getting some calls, asking why don't I vote to impeach the president?" Gallego said. "Well, I'm not signing up to impeach the president, because I believe in the impartiality of the special prosecutor. If the report comes out and says something illegal happened, that's when I'll make a decision."
Still, support for impeachment is growing in some quarters. The first time Green brought his push to a vote, 58 House Democrats supported it. The second time, 66 backed it — including Democrats who have jumped into races for governor and the Senate.
"It's a huge issue in my district," said Rep. Pramila Jayapal (D-Wash.), a member of the committee who supported the Green resolution. "We get a massive amount of email, mail, all sorts of communication from people who believe that Trump is not fit to be president on a number of levels."
Democrats don't all agree on what constitutes grounds for impeachment. Green said in an interview this month that his push is a response to the president's rhetoric, which he sees as discriminatory and disturbing. He highlighted Trump's proposed Muslim ban, his remarks in the wake of the deadly violence in Charlottesville and his transgender military ban.
Also, Nadler's record and philosophy suggest that he will not necessarily push relentlessly for impeachment.
"An impeachment cannot be partisan," he said at the Capitol earlier this month, explaining that a majority vote for impeachment in the House isn't much use if it is likely to fail in securing the two-thirds vote required in the Senate.
"There isn't a hell of a lot of a point," he said, "to impeaching a president and having him acquitted in the Senate."
More importantly, Nadler said, some portion of those who voted for the target of impeachment must support the effort, or at least believe "you had no choice."
"If you don't achieve that, then the country's going to be torn apart. You're going to have 20 years of recriminations. 'We won the election, you stole it.' "
Nadler replaces John Conyers Jr. (Mich.), who resigned in December, creating an election between Nadler and Rep. Zoe Lofgren (D-Calif.). A campaign leaflet, distributed to House Democrats, called Nadler "our strongest member to lead a potential impeachment." Lofgren had been on the staff of the Judiciary Committee when it investigated Nixon; Nadler had known Trump for more than 30 years, since he was a state assemblyman whom the real estate mogul tried, and failed, to win over. Nadler won, with around two-thirds of his colleagues supporting him.
"The vote for Jerry Nadler was a vote saying, 'We need to be prepared to impeach this president,' " Steyer, the Democratic mega-donor, said in an interview. "And he's the person best positioned to do it in an evenhanded and thoughtful and professional way."
The subject of impeachment has recurred in Nadler's career every 10 years, a sort of political cicada, stirring from hibernation and making a familiar noise. His first encounter came in 1998, a year that the House Judiciary Committee devoted almost entirely to the investigation of then-President Bill Clinton's extramarital affairs.
Nadler, a backbencher then in his third term, became a sort of liberal star. Witty and at the time rotund — he would undergo lap band surgery in 2002, dropping 100 pounds from a 5-foot, 4-inch frame — he brawled with Republicans from the committee to town hall meetings to the lights of cable news.
The impeachment push, Nadler said in 1998, amounted to a "coup d'état," a mangling of a misunderstood constitutional power. "The purpose of the impeachment provision was precisely, if you read the Federalist, to protect American liberty against the encroachments of a chief executive who would abuse his or her power to encroach upon liberty," Nadler said in one floor speech.
Democrats lost the vote, but won the political argument, with the midterm electorate of 1998 breaking in favor of the president's party for the first time in 64 years. Clinton's impeachment trial in the Senate was perfunctory, ending in a bipartisan acquittal.
In 2008, Nadler switched roles, but he did not switch his theory of impeachment. Democrats gained the House in the final midterm of George W. Bush's presidency. A restless liberal base had spent years asking for impeachment hearings, inspired by everything from the launch of the Iraq War to spying programs enabled by the Patriot Act.
Nadler, a persistent critic of Bush, found himself fending off cries for impeachment.
Nadler's record in those fights helped his reputation with Republicans. It also helped that the libertarian-leaning Republicans on the committee found him to be a reliable, knowledgeable ally on civil liberties.
"He wouldn't jump to impeach," said Rep. Darrell Issa (R-Calif.). "He's a pretty reasonable guy who isn't going to use that unless there's some effort underway already. I don't expect him to pull punches, but I also don't expect him to swing at everything that goes by."