But Ms. Collins has suggested all along that she will do just that.
“I tend to like information,” she said on Monday, adding that she would not be pushing for the option to call witnesses “if I did not anticipate at the end of hearing the case presented, and the Q. and A., that there might be a need for more information.”
The names of other Republican senators — including Cory Gardner, who is in a tough 2020 race in Colorado, and Lamar Alexander of Tennessee, who is respectful of Senate institutions and is retiring and thus freer to vote his conscience — have also been raised as possibilities. Mr. Gardner can ill afford to break with Mr. Trump.
But Mr. Alexander is of genuine concern to the White House, according to a senior administration official, who insisted on anonymity to characterize the perspective of Mr. Trump’s team. On Monday, he appeared to join the Collins-Murkowski wait-and-see camp.
“We’re taking an oath to be impartial,” Mr. Alexander told reporters in the Capitol, “and that to me means we have a constitutional duty to hear the case, ask our questions and then decide whether we want additional evidence in terms of documents or witnesses.”
After the House voted last month to impeach the president on charges of high crimes and misdemeanors in connection with his campaign to pressure Ukraine to investigate his political rivals, Mr. Schumer issued a list of four witnesses Democrats want to call. Those include Mr. Bolton and Mick Mulvaney, the acting White House chief of staff.
Mr. Bolton has since said he would be willing to testify if the Senate issued a subpoena.
Democrats also want to subpoena documents, including administration emails showing that the White House ordered the suspension of military aid to Ukraine just 90 minutes after a phone call in which Mr. Trump asked that country’s president to investigate Joseph R. Biden Jr. and his son, Hunter Biden.
Mr. Trump, who has vacillated between saying he wants witnesses and saying he wants a speedy trial, has at times floated the idea of calling Hunter Biden to testify. If Democrats succeed in calling Mr. Bolton or other witnesses, it is likely that Republicans would push to call the Bidens. On Tuesday, Mr. McConnell met with a small group of Republican senators to explore that idea of “witness reciprocity,” according to two people familiar with the discussions.