Sen. Debbie Stabenow (D-Mich.) argued, “We need to do the right thing regardless of who’s benefiting,” before noting that there is an outsize number of Republican seats up for grabs next year.

A Senate trial “will make it make it very difficult for them to choose,” added Stabenow, the No. 4 Senate Democrat. “If I were them in many of the states where people are running, I certainly would, politically, be concerned about taking that vote.”

Democrats need to net at least three seats to win back the Senate, provided they also win back the presidency. And they have several opportunities to do so, but it will mean winning at least some races in states Trump carried in 2016.

Democrats consider the most vulnerable Republican senators to be Cory Gardner of Colorado, Martha McSally of Arizona, Susan Collins of Maine and Thom Tillis of North Carolina, with several others viewed as reach-states that could become increasingly competitive or fall off the map altogether depending, in part, on Trump’s popularity.

“It’ll be a very problematic vote in a lot of states,” said Senate Minority Whip Dick Durbin (D-Ill.). “Public sentiment has shifted. The polling numbers suggest much stronger support for pursuing an impeachment inquiry.”

Republicans largely dismiss the notion that impeachment poses a political threat.

"The idea that this is a political winner for Democrats is total fiction. Having three of the most liberal senators say differently doesn’t make it true," said Kevin McLaughlin, executive director of the National Republican Senatorial Committee. "This strikes me as nothing more than an attempt by liberal members to bully others in their party."

Some GOP senators and the Senate Republican campaign arm have also fundraised off impeachment to rile up the conservative base. And so far, most vulnerable Republicans are striking a defiant tone.

“To me it’s not a hard vote. The facts lead you where they lead. What I’ve seen to this point makes me wonder if it’s going to be anything other than a political exercise,” Tillis said. “Having the Democrats on record for a frivolous activity on an impeachment vote may be a hard vote for them.”

National polls show an uptick in support for an impeachment inquiry amid the Ukraine firestorm, but how that will translate to the politics of an impeachment trial in the Senate won’t be clear for months.

“Having the Democrats on record for a frivolous activity on an impeachment vote may be a hard vote for them.” Sen. Thom Tillis

Most Democratic challengers to GOP senators also backed the House opening an impeachment inquiry against Trump but have been more cautious and measured in their statements on it.

Senate Democrats are also protecting incumbents in two states that Trump won in 2016 for whom an impeachment vote could be challenging: Doug Jones of Alabama and Gary Peters of Michigan. Both senators backed the House impeachment inquiry but expressed caution on handicapping the outcome. In a speech on the Senate floor on impeachment, Jones angrily said, “partisan tribalism is taking over already.”

Democrats recognize they need to be cautious in how they talk about trying to oust the president: It does them no good to be seen as pursuing impeachment for political gain. Sen. Ben Cardin (D-Md.) said impeachment “should not be a political consideration at all,” while Sen. Kamala Harris (D-Calif.), a presidential contender, said, “This is a time to put country over party, and I hope that people have the courage to do that.”

“I'm concerned, I think like many people, that we have a president of the United States who calls a president of a foreign country and asks them to investigate his political rival,” said Sen. Catherine Cortez Masto (D-Nev.), chair of the Democratic Senatorial Campaign Committee. The DSCC has done little explicit impeachment messaging in recent days.

Battleground state Republicans largely brushed off a transcript of Trump’s conversation with Ukrainian President Volodymr Zelensky, in which Trump urged Zelensky to probe former Vice President Joe Biden, one of his main rivals in 2020. But few commented after the subsequent release of a whistleblower report documenting more alleged malfeasance.

Democrats say it’s only going to get worse as the inquiry reveals more of the president's conduct.

“It’s going to be much more difficult than they are suggesting right now,” said Sen. Sheldon Whitehouse (D-R.I.), a former prosecutor. “It’s really hard, I think, if you’re in their shoes to hold out for very long the pretense that the transcript of the Zelensky phone call was exculpatory. As a guy who spent part of his life listening to organized crime wiretaps, you don’t get much better than that.”

As they prepared to head home and regroup for a two-week recess, Republicans tried several different approaches to ease the pressure.

“The House ought to quit messing around and just go ahead and vote." Sen. John Cornyn

Gardner called for a search for the facts while Collins insisted that she can’t comment on daily events when she may serve as a juror in a conviction trial. Tillis and McSally moved to quickly dismiss questions of impeachment, with McSally predicting it would backfire on Democrats.

And Sen. John Cornyn (R-Texas), whose race could be competitive, basically urged Speaker Nancy Pelosi to send him impeachment articles so the Senate can dismiss them.

“The House ought to quit messing around and just go ahead and vote,” he said. "I'm not going to vote to convict somebody without any evidence."

When asked about the politics of the situation, most Republicans will say it’s an overreach by Pelosi that will backfire on Democrats. Sen. Lindsey Graham (R-S.C.), who was a leading advocate for impeaching former President Bill Clinton, said, “We all thought we were right and the public thought we were a little crazy.”

It is possible Senate Majority Leader Mitch McConnell (R-Ky.) could try to minimize an impeachment trial in the Senate through parliamentary procedures. Yet most Republicans believe the Senate is going to respond to any impeachment finding of the House with some sort of vote.

“They have to go on record,” said Sen. Richard Blumenthal (D-Conn.). “History will condemn them if they duck and run.”

For now, the two parties are engaged in a sparring match, with Senate Republicans ridiculing Pelosi for allegedly putting her own majority at risk, with Senate Majority Whip John Thune (R-S.D.) warning "it could backfire” on Democrats.

But some Democrats say such statements smack of weakness. And they are eager to see how Republicans deal with the issue when they are forced to take a stand on the Senate floor.

“Senate Republicans win if they make House Democrats lose their nerve,” said Adam Jentleson, a former top aide to former Senate Majority Leader Harry Reid (D-Nev.). “But all Democrats win if the House impeaches Trump over Ukraine and forces Senate Republicans to decide whether to let a criminal remain in office.”

