Final negotiations on merging the bills began in earnest Wednesday as congressional leaders spent more than eight hours behind closed doors at the White House using a process that bypasses a formal conference committee and cuts out Republican participation.

They dug into a series of sticky issues that separate the two versions of the bill passed by the House and the Senate — including abortion, access to health insurance by illegal immigrants, and how to pay for the sweeping legislation, which will insure an estimated 37 million Americans over the next decade.

A White House statement said they had made “significant progress in bridging the remaining gaps” between the bills.

Republicans have been howling about the process for nearly two weeks — House Minority Leader John Boehner, R-Ohio, called it “a breeding ground for more of the kickbacks” — but now congressional leaders are facing a growing mutiny within their own ranks.

Wednesday, Sen. Mark Udall, D-Colo., suggested the talks lacked transparency and called for televising final negotiations. Sen. Michael Bennet, D-Colo., also said final talks should be open to live cameras.

“I haven’t been in Washington long, but one thing I can tell you is this is one town that can use some fresh air, and some real transparency,” Bennet said. “The outcomes from the negotiations on the health care reform bill are far too important to be done behind closed doors.”

Those criticisms could create significant difficulties for efforts to finish the health care bill and move on to other legislation before the press of this year’s election cycle swamps Congress.

Media organizations and others have said televising final negotiations between the House and the Senate would be natural given the enormous potential impact of the legislation.

Instead, Democrats have created a process under which House and Senate leaders will hammer out key compromises, then send the retooled bill to both chambers.

Those negotiations are expected to go for days, if not weeks, and Democratic leaders have said a conference would only provide Republicans — only one of whom voted for the bills in either chamber — a new chance at obstruction.

“This non-conference conference was a bid to speed this up and bring it to an end. A conference committee seemed one more area where Republicans would attack the Democrats and liberals would get upset with leadership,” said Julian Zelizer, an expert on Congress at Princeton University.

“The problem is it looked bad. Republicans can use that to say health care is being rushed through and done behind closed door because Democrats want to hide something,” he said. “That’s why you’re seeing Democrats now getting nervous about the process as well.”