WASHINGTON — House Democrats sent bills to the Senate only to watch them wither under dismissive Republican leadership on scores of occasions this year. They did so yet again on Wednesday, but this moment carried a weight all its own.
This was no election security measure or minimum wage increase or drug pricing legislation. It was two articles of impeachment against Donald John Trump, a historic condemnation of the 45th president of the United States, a polarizing figure who nonetheless remains popular in the districts of dozens of Democrats who provided the party the House majority that enabled the action against him.
The effort to remove the president from office carries inherent political risks. But they are particularly wrenching for Democrats knowing that their votes will not, barring some entirely unexpected turn of events, lead to a Senate conviction of Mr. Trump and his immediate departure from the White House. They are putting themselves on the line not for an outcome but for a principle.
In the end, Democrats concluded, with almost no defections even from the moderates whose re-elections will be most in jeopardy, that they could not let Mr. Trump’s conduct in encouraging Ukraine to investigate a political rival go unpunished even if it cost some lawmakers their seats. And history suggests that high-profile partisan House initiatives that go on to die in the Senate can yield just that outcome.
“This is about the rule of law, the same rule of law that keeps our money in the bank and keeps our streets and homes safe,” said Representative Joe Cunningham of South Carolina, a freshman Democrat who will have to defend his vote back home in his conservative district. “If you start to lose one corner of the rule of law, all segments of the rule of law are lost. So are you going to stand for the entire rule of law or stand for nothing?”
Led by Speaker Nancy Pelosi, Democrats have emphasized the factual basis behind the vote and urged members to cast it as a matter of constitutional principle. Party leaders said they did not squeeze lawmakers as they might on other tough votes but encouraged them to reach their own conclusions.
“The greatest protection these members will have,” said Representative Steny H. Hoyer of Maryland, the No. 2 House Democrat, “is that they vote their conviction and the Constitution, not their politics.”
Democrats are confident that they built a strong factual case for Mr. Trump’s removal. In weeks of testimony, they documented that he had sought to pressure Ukraine to make a public commitment to carrying out investigations that could benefit him politically.
They showed that Mr. Trump and his aides and allies linked a sought-after Oval Office meeting for Ukraine’s new president to the investigations. And they established that the administration had frozen $391 million in military aid to Ukraine — an ally under threat from Russia — during the period when Mr. Trump was pushing hardest for the investigations.
The risks now, Democrats asserted, rest with Senate Republicans if they fail to give serious consideration to convicting Mr. Trump.
“I’m stunned by some of the reactions, from their own mouths, from the leaders of the Senate abandoning their oath, in defiance of their oath, in plain view,” said Representative Madeleine Dean, Democrat of Pennsylvania, who sits on the Judiciary Committee. “I think of it as malpractice, but it’s much more serious than that.”
Still, the political ramifications for House Democrats were evident, especially with the start of what will certainly be a combative election year just days away.
Led by the president and his allies, Republicans promised to make vulnerable Democrats pay for their impeachment votes and said the partisan split — combined with the near certainty that the Senate would ultimately, and perhaps quickly, acquit the president — would allow them to make the case that this was strictly a political exercise born out of spite against a duly elected president.
Moments after the House voted to impeach Mr. Trump, American Action Network, a conservative advocacy group, announced it would hit Democratic lawmakers who supported impeachment and represent districts won by the president in 2016 with a combined $2.5 million of television and online advertisements. Conservative groups had already flooded those districts with a torrent of advertisements.
“Each and every one of these members will have to explain their vote to impeach President Trump,” Dan Conston, the president of American Action Network, said in a statement. “Folks at home expect their member of Congress to deliver on real issues. Instead, they’ve spent every waking moment trying to remove Trump from office for their own partisan political ends.”
Democrats, on the other hand, needed to satisfy their own core supporters outraged by Mr. Trump’s conduct. Many Democratic voters had demanded that the House take some steps against him even if it was futile, given the steely control over the Senate by Mitch McConnell, the Republican leader, who has made clear he is working in tandem with the White House on impeachment.
Democrats “had to respond,” said Tom Davis, a former Republican House member from Virginia and a top party strategist during the Republican-driven impeachment of President Bill Clinton in 1998. “I think if they had not done it, they may have faced a greater risk,” he said, referring to a possible backlash from the left.
After Mr. Clinton was acquitted on a bipartisan basis in the Senate, Republicans ultimately lost a handful of congressional seats, including one held by an impeachment manager, James Rogan, who was defeated in 2000 by Representative Adam B. Schiff, now the chairman of the House Intelligence Committee and one of the architects of Mr. Trump’s impeachment.
The current divide between the House and the Senate on impeachment is also reminiscent of another politically consequential debate in 2009, when some vulnerable House Democrats took a risky vote on climate change legislation only to see it stall in the more risk-averse Senate, then under Democratic control. That high-profile vote was considered at least a contributing factor in the loss of the House Democratic majority the following year.
Mr. Davis and other experts said it was impossible to assess with any confidence the political consequences, if any, of the impeachment vote given the current political tumult and the political entrenchment of voters. They said they expected at least a few races to turn on the issue, but were uncertain how widespread and sustained the fallout from Mr. Trump’s impeachment would be. In the Senate, moderate Republicans as well as Democrats will face tricky political calculations.
“I don’t know that this is going to be a game changer,” said Nathan Gonzales, a nonpartisan analyst and editor of Inside Elections. “It is clear that Republicans are going to tabulate the hours, days, weeks and months that Democrats spent on this. I think Democrats will have to make a well-funded, concerted effort to educate voters on the other issues they have been working on.”
Democrats are taking steps to make sure voters know that impeaching the president has not been all they have been doing, scheduling a vote for Thursday on a new trade deal with Mexico and Canada so they can exit for the holidays with a more tangible, bipartisan achievement in hand. They are also considering bringing up a measure on state and local tax cuts sought by endangered Democratic lawmakers in California, New Jersey, New York and Maine, among other places.
Party leaders also believe that a decision by Senate not to take the House impeachment seriously could help Democrats by energizing their voters come November when the anti-impeachment fervor might have died down among Republicans. Mr. Hoyer predicted that House Democrats would gain seats in the next election.
And vulnerable Democrats believe they can survive by doing what they see as the right thing.
“I realized that what I had to do was what the Constitution demanded and what our oath demanded,” said Representative Kendra Horn, a Democrat who represents a Republican-leaning district in Oklahoma. “And that was take the vote.”
Republicans spent Wednesday on the House floor assailing Democrats for what they saw as an illegitimate assault on the president and warning they would suffer the consequences. “This will be their legacy,” said Representative Kevin McCarthy of California, the Republican leader.
Even with the possible political jeopardy, most House Democrats seemed willing to live with that outcome.
Catie Edmondson contributed reporting.