WASHINGTON - President Barack Obama's quest to overhaul U.S. healthcare rests in part on the frail shoulders of Senator Edward Kennedy, a champion of the cause now in the second year of fighting deadly brain cancer.

A towering figure among Democrats who has called providing health insurance to all Americans the cause of my life, Kennedy recently floated a bill seen as an first draft of Obama's high-stakes effort.

But whether the Democratic-controlled Congress ultimately passes legislation -- and what it looks like -- may depend on how much time and energy the ailing 77-year-old Kennedy can devote to it in coming months. Conservative Republicans and centrist Democrats are ready to chip away at the more liberal elements of his proposal such as a government-run health plan.

Kennedy is critical to passage (of healthcare legislation), said James Thurber of American University's Center for Congressional and Presidential Studies.

That doesn't mean a bill wouldn't pass without him, Thurber said. Other people are involved. But he's the key. He knows more about healthcare than anyone else in the Senate, he's widely respected by both parties and is one of the most important senators in history.

Obama has called on Congress to pass legislation by October to create a public insurance program that would compete with private insurers, part of his broader plan to overhaul the $2.5 trillion U.S. healthcare system.

This could be a hurdle for Republicans, who along with some private insurers say Obama is setting the country on the road to a government monopoly on health insurance in his bid to provide cover to an estimated 46 million uninsured Americans.

Democrats may have sufficient votes to ram it through the House of Representatives without any Republican support. But Democrats say they want bipartisan Senate backing. Both chambers must pass a bill before Obama can sign it into law.

Draft legislation by Kennedy's Health, Education, Labor and Pensions Committee would require businesses and individuals to purchase medical coverage and bar insurance companies from refusing to cover anyone because of preexisting conditions.

It would also provide federal subsidies to families buying coverage.

DIFFERENT VISIONS

While Kennedy is influential, another player who will determine the final shape of the Democratic health plan is Senate Finance Committee chairman Max Baucus -- who is seen as far more cautious.

With Kennedy largely sidelined by the brain cancer he was diagnosed with 13 months ago, the more moderate Baucus has taken a higher profile on the drive in the past year.

The end product will likely be 75 percent Kennedy, 25 percent Baucus, said a senior Democratic leadership aide. Others say the final balance may be more 50-50.

But as a congressional analyst put it: What everyone knows is that when it comes to cutting a deal, they guy who liberal Democrats have confidence in is Kennedy, not Baucus.

As the leading liberal voice in Congress, Kennedy has been a popular yet polarizing figure and frequent target for conservatives during much of his 47-year Senate career.

Yet a recent survey by The Hill, a Capitol Hill publication, found that Senate Republicans believe Kennedy is the easiest Democrat to work with and the most bipartisan.

That senatorial camaraderie may be put to the test over a big healthcare bill largely because of questions how to pay for it -- it could cost $1.5 trillion over 10 years.

Republican Senator Orris Hatch -- who has teamed up with Kennedy to enact health legislation for the old, young and poor -- said Kennedy's bid to provide insurance to all can happen.

But they have to get together with people like me to get legislation that would be effective, Hatch said.

Some analysts say the final plan must be scaled back to have a shot.

But others say that with the support of elements of private industry, the backing of a popular president and a growing awareness of the size of healthcare in the U.S. economy -- currently 15.9 percent of GDP -- there could be real change on the way and Kennedy could be laying the groundwork.

BEATING THE ODDS?

Obama had first hoped the healthcare drive would be headed by Tom Daschle, a former Senate majority leader and expert in the field initially nominated to become Obama's health czar.

But Daschle dropped out in February amid a flap over his personal taxes, leaving lesser-known figures to guide the White House health effort. For star power, people looked to Kennedy.

Kennedy has made infrequent trips to the Capitol since falling ill, but he keeps in contact with Obama, colleagues and staff who have taken lead roles in the drive for reform.

He walks with a cane, balances work with treatment, and often looks tired and drained. Yet colleagues say he remains mentally sharp and determined.

Kennedy's illness kills half its victims within a year and patients rarely survive more than three years.

He has already beaten the odds, a senior senator said, noting Kennedy has survived more than a year. Nobody wants to openly talk about this, but we all want to get this bill done in time for him to see it signed into law.

(Additional reporting by Donna Smith, Editing by Andrew Quinn and Frances Kerry)

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