Rep. Ben McAdams, D-Utah, said at a news conference that he couldn't "turn a blind eye" to Trump's Ukraine scandal, and Rep. Andy Kim, D-N.J., tweeted his support for the two articles of impeachment — on abuse of power and obstructing Congress.
The wave, which began with committee-level votes from members of the panels investigating the president in previous weeks, ramped up Monday, all but ensuring that Trump will become the third president impeached by the House. Because of four vacancies, the total number of current House members is 431 — meaning 216 votes constitute a majority. There are 200 House Democrats from districts Hillary Clinton won in 2016, and Rep. Justin Amash, a Republican turned independent, favors impeachment.
By early evening Monday, 15 of the 31 Trump-district Democrats had committed to voting for impeachment, with only Jeff Van Drew of New Jersey, who is expected to switch to the Republican Party in the coming days, and Collin Peterson of Minnesota having indicated that they plan to vote no.
By midday Tuesday — with lawmakers in swing districts repeating the sentiment that, regardless of the politics, they would vote to stop Trump from trampling the Constitution — the number in favor was up to 26, with only Reps. Jared Golden, D-Maine, Ron Kind, D-Wis., and Cheri Bustos, D-Ill. undecided.
Let our news meet your inbox. The news and stories that matters, delivered weekday mornings.
The pre-vote announcements have underscored several political dynamics at play for these Democrats: They won in 2018 after Trump campaigned against Democrats on the threat that they would impeach him. It's not clear yet just how big a role impeachment will play in their re-election bids. And there's a real risk that more voters would be lost than gained by crossing the partisan aisle.
For all of those reasons, along with their personal convictions, it was not a long leap from "undecided" to "yes" on impeachment, even as House Republicans lobbied some of those Democrats to vote "no."
"You can measure a shift in the mood and temperature in the hearts and minds of the voters" that resulted in Democrats' winning election in districts where Trump prevailed two years earlier, said Antjuan Seawright, a Democratic strategist based in South Carolina. "While Republicans will try to use impeachment as a fear and a scare tactic, at the end of the day we have demonstrated our ability to legislate and investigate."
Seawright is correct that Republicans think they can turn the push to remove Trump from office into a way to drive Democrats from their seats.
"By supporting impeachment, Chris Pappas has lit his electoral hopes on fire and shown New Hampshire voters he can't be trusted," Michael McAdams, a spokesman for the National Republican Congressional Committee, said after Pappas, a freshman Democrat, came out against Trump.
Brad Parscale, Trump's campaign manager, recently tweeted polling from the district of Rep. Kendra Horn, D-Okla., that suggested that impeachment was unpopular among her constituents. But it also indicated that, even though Trump won by nearly 14 percentage points in the district, 45 percent of voters there favored impeaching him.
Polling results on the issue have depended on which questions are asked of which set of Americans, and they have given both parties reason to hope that they will be on the right side of the politics — no matter how history judges them. FiveThirtyEight.com's rolling average Monday night reported that 47.1 percent backed impeachment, while 46.6 percent opposed it, reflecting a slight narrowing of the gap since the House Intelligence Committee began its hearings on the Ukraine scandal last month.
But a Fox News poll released over the weekend reported that, by 60 percent to 24 percent, voters believe it was wrong for Trump to seek a foreign nation's assistance in investigating a political rival. That raises the possibility that some voters will disagree with Democratic lawmakers' conclusion that Trump should be impeached but that they will also have some sympathy for the decision.
With the election nearly a year away, it's still not clear how big a role impeachment will play in each House race.
Rep. Matt Cartwright, D-Pa., said he didn't factor that in when he arrived at the decision to vote for impeachment.
"It wasn't a consideration for me," he said. "The only oath I took was to defend our Constitution. I'm sworn to do it, even when it's unpopular."
For many of the Democrats in swing districts, however, the politics may align best with voting to impeach Trump anyway, said Jesse Ferguson, a former deputy director of the Democratic Congressional Campaign Committee.
"The wrong decision for a swing district member would be to pretend to oppose something when voters believe they support it," he said. "You can't fake it."
CORRECTION (Dec. 17, 2019, 7:40 a.m. ET): An earlier version of this story mislabeled Oklahoma Rep. Kendra Horn's party affiliation. She is a Democrat, not a Republican.
Jonathan Allen is a senior political analyst for NBC News, based in Washington.