The White House is preparing for defections from Republican senators when it comes time to vote on calling witnesses to testify in the Senate impeachment trial, according to a new report.

Per CBS News, senior White House officials have identified Sens. Lisa Murkowski (R-AK), Susan Collins (R-ME), Mitt Romney (R-UT) and Cory Gardner (R-CO) as likely candidates, with Sens. Rand Paul (R-KY) and Lamar Alexander (R-TN) as additional possibilities in the mix.

Last week, the Bangor Daily News reported that Collins was coordinating with a “fairly small group” of Republicans to come to an agreement that would allow witnesses to be called.

Senate Majority Leader Mitch McConnell (R-KY) intends to begin the process without deciding on the witness question, leaving that vote until later.

Democrats would need four Republicans to join them on a vote to alter the rules to allow witness testimony. This issue attracted more scrutiny after former National Security Adviser John Bolton said he would testify if subpoenaed.