Four GOP senators -- enough to kill the legislation -- say they need hard assurances from House Speaker Paul Ryan that a "skinny" ObamaCare repeal bill won't be Congress's final product before they'll vote for it.

Sens. John McCain (Ariz.), Lindsey Graham (S.C.), Ron Johnson (Wis.) and Bill Cassidy (La.) said Thursday evening they are prepared to vote against a skinny repeal unless they get a guarantee it will go to a conference committee with the House.

"There's increasing concern on my part and others that what the House will do is take whatever we pass" and pass it without making changes, Graham said.

"The skinny bill as policy is a disaster. The skinny bill as a replacement for ObamaCare is a fraud," he said. Graham added that if the skinny bill passes the House, the GOP will own the collapse of ObamaCare.

"I'd rather get out of the way and let it collapse than have a half-assed approach where it is now our problem," he continued.

If the skinny bill were to become law, insurers and other healthcare groups have warned it could cause premiums in Obamacare's insurance markets to skyrocket.

Graham said a conference committee would buy time for the Congressional Budget Office (CBO) to score proposals from himself and Cassidy, as well as by Sen. Rob Portman (Ohio) and Ted Cruz (Texas).

When asked how he'd know any assurance from House leadership is ironclad, Graham said, "It's like pornography, you know it when you see it."

If the Senate does pass a bill, both chambers would have to approve whatever emerges from the House-Senate conference. Alternatively, the House could simply approve the Senate's skinny bill -- a prospect that prompted the press conference from the four GOP senators.

Nothing has been determined yet in the House; GOP leaders appear to be keeping their options open and could call up the bill any time between now and Tuesday.

Senate Majority Whip John Cornyn (R-Texas) said he has had no formal assurances from the House, but has every expectation a conference committee will be called.

"There has been some communication in which it has been reported to me that Paul Ryan said the House is preparing to go to conference," Cornyn said, noting, "I have not talked to Paul Ryan. The Senate doesn't determine whether the House goes to conference."

A spokeswoman for Ryan said the House hadn't yet decided its course of action.

"Conference Committee is one option under consideration and something we're taking steps to prepare for should we choose that route after first discussing with the members of our conference," AshLee Strong said.

GOP leaders have not yet revealed their scaled-down measure, but it is expected that it would repeal the employer and individual mandates from ObamaCare.

In another key provision to win conservative support, it would also block federal funding for Planned Parenthood, which provides abortion services. The skinny bill could also cut ObamaCare's prevention and public health fund, while adding money for community health centers, sources said.

It would not cut Medicaid, which has been a concern for centrist Republicans. However, conservatives could add the Medicaid cuts back into the bill during a conference committee.

It's not clear when the Senate will take a final vote on the skinny measure, but Republicans can only afford two defections and still rely on Vice President Pence to break the tie.

The bill is expected to be the final amendment at the end of a marathon voting session beginning Thursday evening.

House Majority Leader Kevin McCarthy's (R-Calif.) office has warned House lawmakers that they should not immediately depart Washington on Friday, in case the Senate does approve a repeal bill.