CBS news report (slight rewritten): Senior White House officials tell CBS News they increasingly believe that at least four Republicans will vote to call witnesses. But what if Senate witnesses are approved and Republicans insist on calling Typewriter Joe to testify?

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.

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.

White House officials [have] reiterated the president’s intention to claim executive privilege if necessary to block John 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.