Senior White House officials tell CBS News they increasingly believe that at least four Republicans, and likely more, will vote to call witnesses. In addition to Senators Lisa Murkowski of Alaska, Susan Collins of Maine, Mitt Romney of Utah and possibly Cory Gardner of Colorado, the White House also views Rand Paul of Kentucky as a “wild card” and Senator Lamar Alexander of Tennessee as an “institutionalist” who might vote to call witnesses, as one official put it.

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Last week, Collins said she was working with a “fairly small group” of GOP senators to allow new testimony, adding that her colleagues “should be completely open to calling witnesses.” Romney has expressed an interest in hearing from former national security adviser John Bolton, who has said he would testify under subpoena. Murkowski said last week that the Senate should proceed as it did during the 1999 Clinton impeachment trial.

Gardner and Alexander have both said the Senate trial should be fair and impartial. Paul has said the president should be able to call his own witnesses, including the whistleblower whose complaint about Ukraine sparked the impeachment inquiry in the first place.

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