Washington — The White House is preparing for some Republican senators to join Democrats in voting to call witnesses in President Trump's impeachment trial, which could get underway in the coming days.

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.

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.

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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.

Senate Majority Leader Mitch McConnell has said the question of whether to call new witnesses in the trial would be decided by the full Senate after the trial gets underway. A simple majority of 51 votes will be needed to approve motions to call witnesses, meaning Democrats would need to convince four out of the 53 Republicans in the Senate to vote with them to compel testimony.

The House is expected to vote this week on a resolution to name impeachment managers and transmit the two articles to the Senate, a necessary step before the trial can begin. Democrats had previously demanded McConnell agree to allow testimony from witnesses, including four administration officials, before transmitting the articles.

The White House officials, who were not authorized to speak publicly, reiterated the president's intention to claim executive privilege if necessary to block Bolton from testifying. Mr. Trump told Fox News last week that he would likely do so to "protect the office." While Bolton could testify about some events that would fall outside the scope of executive privilege, the White House would fight to prevent Bolton from discussing direct conversations with the president.

One senior official said the White House's impeachment team and counsel's office do not expect a quick dismissal of the impeachment articles in the Senate, despite the president's weekend tweet in which he said Republicans should vote to throw the articles out. Some Republican senators have introduced a proposal to change Senate rules and simply dismiss the articles.

"Many believe that by the Senate giving credence to a trial based on the no evidence, no crime, read the transcripts, 'no pressure' Impeachment Hoax, rather than an outright dismissal, it gives the partisan Democrat Witch Hunt credibility that it otherwise does not have. I agree!" the president tweeted Sunday.

White House officials said the optics of a vote to dismiss would be tough for Republicans, but White House lawyers do expect the question of acquittal to come up immediately following opening arguments and periods for written questions submitted by senators.

The president has offered various opinions about how he would like a Senate trial to proceed, while at the same time nodding and deferring to the Senate. He has also said he would like to hear from the whistleblower, along with House Intelligence Chairman Adam Schiff and Joe and Hunter Biden. However, motions to call them as witnesses are unlikely to succeed, even though Republicans hold the majority in the upper chamber.