What they’re saying: “You don’t want to be one of the first four. But no one gives a f*** about the fifth vote,” a GOP senate aide told Axios. “Especially for all of the 2020-ers. If it turns into a free vote, why wouldn’t you vote for witnesses?”

Republicans still see the four most likely to break with the party line as Sens. Susan Collins of Maine, Mitt Romney of Utah, Lisa Murkowski of Alaska and Lamar Alexander of Tennessee.

But Cory Gardner of Colorado, Thom Tills of North Carolina, Martha McSally of Arizona, Rob Portman of Ohio, Joni Ernst of Iowa and Pat Toomey of Pennsylvania would face pressure to follow.

What we’re hearing: Sources close to Senate Majority Leader Mitch McConnell tell Axios that, if it appears that at least 51 senators will vote for witnesses, McConnell and Schumer will likely try to hash out an agreement rather than going straight to a vote.