U.S. President Barack Obama readied a high-stakes plea for healthcare reform on Wednesday, as a Senate Democrat leading bipartisan negotiations said he was ready to push ahead even without Republican support.

Faced with falling public approval ratings, Obama said his televised address to the U.S. Congress at 8 p.m. EDT (0000 GMT) would provide Americans with a much more detailed plan for overhauling the $2.5 trillion U.S. healthcare system in the face of bitter differences.

But after months of bipartisan Senate Finance Committee talks by the so-called Gang of Six negotiators, panel Chairman Max Baucus said it was time to proceed with or without Republicans.

He said he would move forward with a bill modeled after the proposals he distributed to members in recent days, which would levy a fee on insurers to help pay for coverage of the uninsured but would not include a government-run health insurance option, which he said cannot pass the Senate.

The time has come for action and we will act, Baucus told reporters. He said he hoped and expected some Republicans would back the proposal, but added: I am going to move forward anyway.

His fellow Democrats, who have solid majorities in both houses of Congress, have struggled to find agreement on a reform bill in the face of strong opposition from Republicans who argue it amounts to a government takeover of healthcare.

The revamp of the healthcare industry seeks to make affordable health insurance available to most of the estimated 46 million uninsured Americans -- nearly one-sixth of the U.S. population -- and curb runaway medical costs.

The Republican criticism resonated with many Americans worried that the $1 trillion cost of the overhaul would add to the country's mountain of debt, despite White House assertions that it would be fully paid for and mark a big step toward expanding coverage to the uninsured.

Obama's speech marks a new approach in the White House's effort to rebuild support for the overhaul after Republicans took control of the healthcare debate during the summer with a volley of attacks on the Democrat proposals.

White House spokesman Robert Gibbs said Obama would discuss reforming medical malpractice lawsuits, which Republicans blame for raising medical costs, and the need for a public option alongside private insurers to provide choice and competition.

The president will talk about meaningful malpractice reform tonight. What I hope that does is cause Republicans to understand that we're close to getting something truly significant done for the American people, truly significant for those struggling with the high cost of health insurance, Gibbs said on Fox Morning News.

But Gibbs would not say whether Obama would propose capping malpractice claims, as many Republicans want.

Shares of top health insurers, which have been volatile as the healthcare debate gathers steam, were slightly higher in afternoon trade on Wednesday ahead of Obama's speech. Shares of the S&P Managed Health Care stock index were up about 1.6 percent while the Morgan Stanley Healthcare Payor stock index was up about 2 percent.

DEFINE HIS PRESIDENCY

Obama's success or failure in getting Congress to pass comprehensive healthcare reform this year could help define the rest of his presidency and play a critical role in next year's congressional elections.

If he fails to push through change on an issue that was a centerpiece of his election campaign last year, he would be politically weakened and would likely struggle to get the rest of his ambitious legislative agenda through Congress.

Past failed attempts at healthcare reform include one spearheaded by President Bill Clinton in the 1990s.

Three House of Representatives committees and one Senate committee have completed their work on a healthcare bill. But the focus is on the Senate Finance Committee where Democrats are trying to craft a compromise that will draw at least some Republican support.

Senator Ken Conrad, a Democratic member of the Gang of Six, told reporters the idea of a bipartisan compromise was not dead. Other Democrats attending the session said they still favored a public insurance option and wanted to see the Finance panel's full proposal before deciding.

The plan includes sweeping insurance market changes. It calls for non-profit cooperatives to compete with insurance companies rather than the government-run public option sought by many liberal Democrats and backed by Obama.

Obama told ABC he would use his speech to make sure that Democrats and Republicans understand that I'm open to new ideas, that we're not being rigid and ideological about this thing, but we do intend to get something done this year.

He dodged repeated questions on whether he would veto a healthcare bill that did not include a public plan.

There are principles that if they are not embodied in the bill, I will not sign it, he said.

The bill should not increase the deficit, should expand healthcare coverage to the uninsured and include insurance reforms, he said.

(Additional reporting by Ross Colvin, editing by Eric Beech and Vicki Allen)

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