As Obamacare repeal talks enter crunch time with a vote as soon as this month, the Senate bill continues to tilt toward more moderate members of the GOP on keeping some of Obamacare’s regulatory structure and providing a more generous wind-down of the law’s Medicaid expansion. The movement has made Republicans increasingly pessimistic that two critical conservative senators, Mike Lee of Utah and Rand Paul of Kentucky, will be able to vote for the GOP’s ultimate agreement on health care, according to senators and aides.

“I think [Lee is a no]. And Rand will be a no,” said a Republican senator granted anonymity to discuss sensitive internal conference matters.

Losing those two senators would be a major blow that would allow Senate Majority Leader Mitch McConnell no further defections in his 52-senator majority and make Sens. Susan Collins of Maine and Lisa Murkowski of Alaska the key swing votes, further imperiling conservatives’ negotiating position in a Senate in which McConnell needs 50 votes at a minimum.

Worry is increasing among conservatives inside and outside the Capitol that the bill is “tipping toward the moderates,” said a Republican working on the repeal effort. And after weeks of sparring, the tug-of-war between conservatives and more centrist Republicans is finally reaching its climax.