The disparate group’s views on the trial are a concern to the White House and GOP leaders, who are worried some could break and vote with Democrats on key trial-related issues, sources tell CNN.

If four of them were to buck calls from GOP leaders for a short, witness-free trial, it could upend the process and create the kind of wild uncertainty Senate Majority Leader Mitch McConnell says he has been carefully “coordinating” to avoid in ongoing talks with top White House officials.

The group includes moderates up for reelection, like Sen. Susan Collins of Maine, who may want to show independence from Trump; seasoned veterans, like Sen. Lamar Alexander of Tennessee, who are retiring and who may not feel politically bound to support the President; and outright critics of Trump, like Sen. Mitt Romney of Utah, who have challenged his unorthodox presidency and who may want to learn more about the allegations of a quid pro quo with Ukraine that is at the heart of the impeachment.