WASHINGTON – As Senate Republicans barrel forward with another, possibly final attempt to repeal and replace the Affordable Care Act, the bill’s architects are staying cautious in case it ends in failure.

The bill is being pushed through the Senate using reconciliation, which allows Republicans to bypass a filibuster and pass the legislation with a simple majority. In a Senate where Republicans have only a two-vote majority, any defectors can derail the process as they did during the latest attempt in July.

Sen. Bill Cassidy, of Louisiana, who is one of the bill’s coauthors, said he could not determine the vote count or whether it was likely to pass.

“I’m still cautiously optimistic, but there’s a lot of moving parts,” Sen. Ron Johnson, of Wisconsin, another of the bill’s architects, said. “It’s kind of coming together.”

Sen. John McCain, of Arizona, who was one of the deciding votes against the previous effort to repeal the ACA, also known as Obamacare, told reporters he was undecided on the bill. McCain said that while he was not waiting for an official analysis from the nonpartisan Congressional Budget Office, he wanted the bill to be passed through regular order.

“I am going to continue to look at this as the process goes on,” McCain said. “I want a regular order, and that’s what I said a week ago – two weeks ago.”

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While a handful of moderate Republicans are still mulling their support, the only vocal GOP senator to come out strongly against the bill is Rand Paul of Kentucky.

“You know I’ll do everything I can to describe this as keeping Obamacare, Obamacare lite,” Paul said. “But I oppose it, and we’ll see what happens.”

Senate Democrats are left on the sidelines as Republicans whip the votes in their own conference. Through reconciliation, they have few, if any, options to block the bill.

Senate Minority Leader Chuck Schumer, however, said in a press conference earlier Monday that he was exploring any ways his conference could prevent the bill, which was also coauthored by Sen. Lindsay Graham of South Carolina, from passing.

“Look, this is so outrageous and so harmful that we’re going to look at every possible way to slow this bill down,” Schumer said.

Among other things, Schumer wants a concrete score from the Congressional Budget Office, which he believes would shine a light on the ramifications of the bill if it were to become law.

“It would be outrageous for our Republican colleagues to vote for this bill without knowing its effect on people,” Schumer added. “That – whatever your ideology – would be nothing short of a disgrace.”

But the CBO announced in a statement Monday that it would be able to provide only a preliminary score of the bill, without any “point estimates of the effects on the deficit, health insurance coverage, or premiums for at least several weeks.”