Senate GOP leaders acknowledge that Romney is pushing his position, but so far, they publicly argue the dynamic inside the Republican Conference has not changed despite the Bolton revelations.

“It’s not a new position for [Romney]. He’s been on that position for quite a while,” said Sen. John Barrasso of Wyoming, the No. 3 Senate Republican. “He didn’t say anything new at lunch that he hasn’t said before.”

If the effort to subpoena Bolton moves forward, Republican leaders will respond with their own explosive push to call Hunter Biden or another witness favored by the White House. Other Trump allies are also echoing this line, declaring that if Bolton is called, then “the floodgates are opened.”

“I don’t see the need to have more witnesses unless we have a lot more witnesses,” said Sen. Kevin Cramer (R-N.D.). “I don’t know what the country would gain from that.”

Romney sent shock waves through the Capitol when he said on Monday morning that it’s “increasingly likely” that more Republicans would embrace his call for witnesses.

Among the senators most likely to join Romney, Collins and Murkowski are Toomey, Lamar Alexander of Tennessee, Rob Portman of Ohio or Bill Cassidy of Louisiana, according to GOP sources. But none have taken the public plunge.

Barrasso said the witness vote — set for Friday — "is still going to be close. They need four. And I haven't seen anybody shift."

Romney said later Monday evening that he didn’t base his earlier statement on any inside intel. He just thinks that if Bolton is willing to talk, logically, Republicans should be willing to listen.

“My sense was that based upon the fact that there was apparently relevant information, material information that others would say: ‘Yeah, OK, that’d be interesting to hear if we could,’” Romney said.

Heather Caygle and Marianne LeVine contributed to this report.

Read More