WASHINGTON - 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.

The policies are not matching up to the promises. They're loading trillions of dollars of debt onto the American people, DeMint said.

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 (0001GMT) on Wednesday.

The overhaul plans call for a government-run insurance program to compete with private insurers, expand coverage to most of the 46 million uninsured Americans and hold 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.

The House Energy and Commerce Committee canceled a planned drafting session for the second consecutive day on Wednesday in order to work with the fiscal conservatives on the committee, who could scuttle the bill.

Another panel, the House Ways and Means Committee, was to meet to discuss taxes and other roadblocks to 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, will continue a series of closed-door meetings aimed at finding a compromise that could be crucial to getting the proposal through the Senate.

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

The August deadline for passing versions in each chamber of Congress appeared to be slipping, however, 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 rallying during the recess.

(Additional reporting by Tabassum Zakaria; editing by Vicki Allen)