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.