Washington (AFP) - Democrats gearing up for a new round of battles against Republican efforts to do away with Barack Obama's signature health care law are condemning a US Senate replacement bill being crafted by Republicans behind closed doors.

Senator Bernie Sanders urged Democrats on Sunday to do "everything they can" to oppose a Republican bill that for weeks has been drafted by party leaders in secret.

"My understanding is that it will be brought forth just immediately before we have to vote on it. This is completely unacceptable," Sanders, an independent who is a member of the Democratic party leadership, told CBS's "Face the Nation" program.

"It seems to me that what they want to do, because this legislation is so bad, is keep it secret, keep it hidden, and in the last possible second rush it before the Senate and get a vote within a few hours. That is beyond belief," Sanders said in a separate interview with CNN.

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The nonpartisan Congressional Budget Office says that a House version of the bill -- an effort to overturn the Democratic former president's sweeping reforms -- would cause 23 million Americans to lose health insurance.

The measure, which has already cleared the House of Representatives, is now before the Senate.

"It was the worst piece of legislation frankly against working class people that I can remember in my political life in the Congress... Throwing 23 million people off of health insurance is beyond belief," said Sanders, who lost the Democratic presidential nomination last year to Hillary Clinton.

Sanders said the Senate health care overhaul measure is particularly objectionable because of the lengths to which Republicans are going to ensure no one finds out what is actually in the bill.

"In the Senate, what you have is you have I believe it is 10 Republicans working behind closed doors to address one-sixth of the American economy," he said.

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Republican Senator Marco Rubio, however, insisted that the closed talks were only the start of what will eventually become a more public process.

"The Senate is not a place where you can just cook up something behind closed doors and rush it for a vote on the floor," said the Florida Republican, also speaking to CBS on Sunday.

"Ultimately we are all going to see what is in it, and the rest of us will have an opportunity to make changes to it, as a condition of our vote," Rubio said.