WASHINGTON (Reuters) – U.S. President Barack Obama will plead his case for a broad healthcare overhaul in a prime-time news conference on Wednesday, with doubts growing about the plan even among his fellow Democrats and polls showing slipping public support.

Leaders in Congress struggled to find common ground on the cost and scope of a proposal that Obama has made his top legislative priority, and hopes dimmed they could meet his goal of passing early versions by the August summer recess.

The package has run into opposition from all sides, with a group of conservative Democrats questioning the cost and funding, liberal Democrats concerned it would not do enough, and Republicans blasting the more than $1 trillion price tag and seeing a chance for a crushing political defeat of Obama.

"We need to put the brakes on this president. He's been on a spending spree since he took office," Republican Senator Jim DeMint, a conservative who recently said the healthcare debate would be Obama's "Waterloo," told NBC's "Today" show.

Obama has stepped up his involvement in the debate, meeting with rebellious House of Representatives Democrats at the White House on Tuesday and scheduling healthcare events throughout the week, topped by the nationally televised news conference at 8 p.m. EDT (0001 GMT) on Wednesday.

The overhaul plans call for a government-run insurance program to compete with private insurers, expanded coverage to most of the 46 million uninsured Americans and holding down soaring healthcare costs that are rising faster than inflation.

But the details have proven difficult for lawmakers to nail down, and a series of opinion polls show Obama's approval rating dipping and his support on the healthcare issue falling to below 50 percent in a Washington Post poll.

U.S. House Speaker Nancy Pelosi said lawmakers were making "great progress" on healthcare, and said there were enough votes to pass the bill in the House. But she did not give a firm timetable for a vote.

'LONG WAY TO GO'

The August deadline for passing versions in each chamber of Congress appeared to be slipping, and Republicans pushed hard to put on the brakes. Obama wants the first versions of the bills passed before the monthlong break to keep opposition from building during the recess.

"It takes some time, but we're going to do it right," Pelosi told reporters

The House Energy and Commerce Committee canceled a planned drafting session for the second consecutive day on Wednesday in order to work with the fiscally conservative Democrats on the committee known as "Blue Dogs," who could scuttle the bill.

"We are making progress; however, we have a long way to go," said Representative Mike Ross, leader of the Blue Dogs.

Another panel, the House Ways and Means Committee, will meet to discuss taxes and other issues in its version of the bill. Its plan to add a tax on the wealthy, to raise about $544 billion over 10 years, has come under fire.

Members of the Senate Finance Committee, meanwhile, continued a series of closed-door meetings aimed at finding a compromise that could be crucial to getting the proposal through the Senate.

Senate Finance Committee Chairman Max Baucus said leading members of the panel trying to negotiate a bipartisan agreement on legislation were "making progress."

The handful of senators spent the morning looking to find more ways to save money in the Medicaid health program for the poor, Senator Kent Conrad told reporters after the meeting.

The more savings lawmakers can wring from existing health programs, the less new revenue they will need to raise.

"We've gone through a series of options on Medicaid to try to see if there couldn't be additional savings," Conrad said. "The work goes on."

'AFFORDABLE PLANS'

Senator Olympia Snowe, a crucial Republican swing vote, said the senators negotiating the committee's package want to make sure the insurance coverage provided through a proposed exchange program would be affordable.

"If you have an individual mandate you have to make sure that affordable plans are available," she told reporters.

The proposed state insurance exchanges would operate as a clearinghouse where individuals without employer sponsored health insurance and small businesses, with up to 50 employees, could shop for medical coverage plans.

Obama said on Tuesday that the bills were "not where they need to be" but he remained confident that he could win approval.

Senate Democratic leader Harry Reid said taking no action to reform the $2.5 trillion healthcare industry was not an option and accused Republicans of supporting the status quo.

Senate Republican leader Mitch McConnell said he supported reform, but not proposals that would increase the federal debt and hurt small businesses.

"Unfortunately, the administration seems bent on its own proposal for a government-driven plan that costs trillions of dollars and asks small businesses and seniors to pay for it," McConnell said.