WASHINGTON — The likely defection of two Senate Republicans has left their leaders no margin for error as they move on Thursday to unveil another version of their bill to repeal much of the Affordable Care Act — without an assurance that they have the votes even to begin debate next week.

Senator Rand Paul of Kentucky flatly declared on Wednesday that he would not vote to take up the bill, which he said left too much of President Barack Obama’s health law in place. Senator Susan Collins of Maine has gone almost as far, saying that the bill cuts too much and that only major revisions could win her over.

The loss of a single additional senator would doom, at least for now, the Republicans’ effort to make good on their seven-year promise to uproot the health law. Unity is proving elusive. A lunch meeting of Republican senators on Wednesday ended with no clear consensus.

After scrapping plans for a vote last month, the majority leader, Senator Mitch McConnell of Kentucky, wants to try again next week. But senators have shown few signs of substantial progress toward the needed 50 votes.