WASHINGTON — Senate Republicans facing steep re-election races next year know the impeachment inquiry coursing steadily ahead on the other side of the Capitol will determine President Trump’s political fate. Their growing fear is that it will also determine their own.
In a matter of weeks, those few senators have watched helplessly as the investigation by House Democrats has lobbed a grenade into the middle of their campaigns, putting them on the defensive amid a torrent of damaging revelations about the president’s conduct with no clear end in sight.
It has tightened the squeeze they were already feeling between a political base that demands unquestioning loyalty to Mr. Trump, and the moderate swing voters who may very well decide their elections. And it is one reason that support for the president in the Republican-led Senate appears to be subtly softening, a phenomenon that some of his own advisers fear that Mr. Trump does not fully appreciate.
Senator Cory Gardner of Colorado, known as a talented campaigner, abruptly walked away from a filmed interview last weekend to avoid answering a question about the military assistance Mr. Trump withheld from Ukraine, a central issue in the inquiry into whether the president enlisted a foreign government to smear his political opponents. In brisk dashes back to their offices, Senators Martha McSally of Arizona and Joni Ernst of Iowa, quickly pivoted to other issues such as rising health care costs, border security and the trade deal with Mexico and Canada. And Senator Susan Collins of Maine has rebuffed any effort to get her to weigh in on impeachment, saying that doing so could jeopardize her impartiality as a juror in an increasingly inevitable trial of the president.
It is not an attractive prospect for senators already toiling to balance between appealing to a conservative base they badly need to win re-election and drawing the support of more centrist voters who polls show support the impeachment inquiry.
“The potential pitfall for Republicans is that they stay so glued to the president that they alienate too many independent voters for their majority to survive,” said Nathan L. Gonzales, the editor and publisher of the nonpartisan Inside Elections, “Or stay glued to him through anything — that there’s no allegation that they’re willing to break from him on.”
So far, Republicans’ strategy has been to keep attention on the secretive way in which Democrats have handled the inquiry. The main complaint of a resolution introduced on Thursday by Senator Mitch McConnell of Kentucky, the majority leader, and Lindsey Graham of South Carolina, the chairman of the Judiciary Committee, calls the process unfair. But they are also keenly aware that there is an expiration date on that approach, given that Democrats soon plan to hold a series of public hearings to lay out their case, raising the possibility that their bind will only deepen as the more information pours out.
There is no indication so far that Senate Republicans will desert Mr. Trump in large enough numbers to join with Democrats and make up the two-thirds majority required to remove him. But Republicans close to the White House are increasingly concerned that a bloc of senators could emerge, including some they suspect might not run for re-election, to break with the president.
Mr. McConnell has instructed fellow Republicans privately that they must figure out individually the impeachment message that works best for them politically. But he is also keenly focused on tailoring the process to insulate his most vulnerable members from a constituent backlash. The resolution introduced on Thursday was in part an effort to allow Republicans to unite publicly behind a measure critical of the inquiry, a way to show the party base that they were behind Mr. Trump even as they refrained from defending his actions.
Ms. Ernst, Ms. McSally, and Senator Thom Tillis of North Carolina were among those who have so far publicly supported the measure.
Senator Todd Young of Indiana, the chairman of Senate Republicans’ campaign arm, said in a brief interview that the focus on impeachment gives senators “an opportunity” to show voters that while they are being “attentive and conscientious to current events, they remain intently focused” on the issues their constituents sent them to work on.
Campaign consultants have stressed to senators the importance of maintaining their own credibility, according to two senior Republican officials, especially given that new revelations may still emerge. They have instructed senators not to respond to every turn of the screw, one reason that most of them have dodged questions about Mr. Trump’s conduct or resorted to complaints about the process.
Mr. McConnell, for his part, offered a stilted if telling response on Tuesday, when a reporter asked him during a diplomat’s testimony this week if he was “willing to defend the president in this matter.”
“I’m willing to talk about the process in the House,” Mr. McConnell said. “I just did. I think it’s grossly unfair and I think the president has a legitimate complaint about the process.”
Some endangered Republicans have also carefully eyed blueprints tested by senators considered to be among the savviest in the conference. Senators Rob Portman of Ohio and Pat Toomey of Pennsylvania, have offered one alternative: staking out the position in interviews with local publications that while Mr. Trump’s call with President Volodymyr Zelensky of Ukraine may have been inappropriate, it is not impeachable.
Of the most vulnerable cluster of senators, only one has offered a full-throated defense of Mr. Trump: Mr. Tillis. But Mr. Tillis also has the most immediate threat to counter: a credible primary challenge from Garland Tucker, a deep-pocketed businessman who has accused the lawmaker of disloyalty to the president in a series of ad buys on Fox News.
“I see nothing there that rises to a level of impeachment,” Mr. Tillis said in an interview this month, describing the account the White House released of Mr. Trump’s phone call with the Ukrainian president. “I’m not going to get into any nuanced questions, or choices, the president made. I’m looking at the plain letter of the transcript, and I see nothing that rises to a level of impeachment.”
For their part, Democratic candidates challenging incumbent senators have largely shied away from using their responses as a vein of attack, though some see the lackluster response and viral video clips as a way to tie incumbents even more directly to the president and sway independent voters.
“There’s only a few voters where this theater may actually matter because Republicans and Democrats have made up their mind when it comes to the president and the Republican Party,” said Mr. Gonzales of Inside Elections. “It’s virtually impossible to know the full scale of the fallout until it’s over.”
Maggie Haberman and Annie Karni contributed reporting.