WASHINGTON – Republican Ron Johnson of Wisconsin joined three other conservatives in the U.S. Senate in withholding their support for the Obamacare replacement bill their party unveiled Thursday.

“Currently, for a variety of reasons, we are not ready to vote for this bill, but we are open to negotiation and obtaining more information before it is brought to the floor,” Johnson, Ted Cruz of Texas, Mike Lee of Utah and Rand Paul of Kentucky said in a joint statement.

“There are provisions in this draft that represent an improvement to our current health care system but it does not appear this draft as written will accomplish the most important promise that we made to Americans: to repeal Obamacare and lower their health care costs,” they said.

Of the four, Johnson’s dissent may be the most surprising, since it has been widely assumed in Washington he would ultimately vote for the bill. And he may still.

But Johnson has for months signaled his reservations about his party’s approach to repealing Obamacare, a program he has long assailed. He criticized the House bill that the Senate bill is patterned after, sharing some concerns with moderate Republicans and other concerns with conservative Republicans. He has repeatedly criticized the legislative approach his party has taken and voiced his differences with leadership over the process.

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In interviews with the Journal Sentinel and other media outlets, Johnson has complained that a vote next week gives him too little time to evaluate the Senate bill and get input from his constituents. He has argued that Congress should worry first about stabilizing the health care markets while taking more time to get a full-blown Obamacare replacement right. He has said Republicans should have tried harder to work with Democrats.

Johnson has also voiced concern over how states like Wisconsin that didn’t join in the Medicaid expansion under Obamacare are treated compared to states that took the federal funding for expanded Medicaid.

On MSNBC’s “Morning Joe” Thursday, Johnson defended aspects of the GOP bill, including the historic rollback of the Medicaid entitlement.

“We’re trying to do a little bit, just a little bit, of entitlement-controlling so we make Medicaid sustainable in the future,” he said.

But Johnson repeated his objection to a quick vote next week on the bill.

“I will insist we have enough time certainly to satisfy myself — and get the input from my constituents — that in the end … this is a continuous improvement from where we are right now,” a system he called  “crazy” and “unsustainable.”

Tammy Baldwin, Wisconsin’s Democratic senator, issued a statement Thursday assailing the GOP Senate bill:

“For Wisconsin families struggling to get ahead, this repeal plan has no heart and people are scared that it will make things worse. It will make families pay more for less care and increase the number of people who are uninsured. The guaranteed protections and care that you have today are weakened and now, politicians in Madison will decide whether you keep the care you have, or whether it is taken away.”

The announcement by Johnson, Cruz, Paul and Lee that they can’t support the bill in its current form underscores how little margin Senate Republicans have for passing a new health care law.

With a majority of 52 members, they can only afford to lose two Republicans, since no Democrat will support their effort to repeal Obamacare.

Even though the four conservatives issued a joint statement about their stance, their individual concerns aren’t necessarily identical. What’s more, it is not only conservatives who have reservations. A handful of Senate GOP moderates have also expressed deep concern about the bill, including its Medicaid cuts.

In the negotiations that unfold over the next week, a concession to one side of the GOP caucus could threaten support from the other. At the same time, Republicans across the board will be under immense political pressure from within their own party to pass a bill.