* Aides promise specifics, not new legislation
* Speech at 8 p.m. (0000 GMT) on Wednesday
* Congressional Democrats optimistic
By Patricia Zengerle
WASHINGTON, Sept 9 (Reuters) - President Barack Obama takes on the bitter healthcare reform debate on Wednesday with a high-stakes speech to the U.S. Congress on his top domestic policy priority.
Aides have promised Obama's nationally televised address will provide specifics about his vision for overhauling the $2.5 trillion U.S. healthcare system -- although they said he will not offer his own legislation.
"The president will outline his plan moving forward," both on healthcare and how to get a bill passed by Congress, said spokesman Robert Gibbs. "I don't think you'll walk away confused about where he is."
Obama told ABC News in an interview that 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."
Elected last year on a platform of change, Obama has proposed a wide-ranging reform of healthcare by cutting costs and expanding coverage to the 46 million Americans without health insurance.
But his fellow Democrats, who have solid majorities in both houses of Congress, have struggled to craft a reform bill while most Republicans have fought it.
Insurance companies, pharmaceutical manufacturers, hospital managers -- and average American patients -- all have huge stakes in how the battle plays out.
Obama's speech marks a new approach in the White House's effort to strike a deal after a summer of sometimes angry words and concern over the scope and cost of the healthcare overhaul dampening Obama's approval ratings in opinion polls.
Its success or failure could help define the rest of Obama's term and perhaps his presidency after Republicans took control of the healthcare debate during the summer with attacks on Democratic proposals during congressional town meetings.
"We're at the point in the legislative debate where he needs to put some things on the table and take some other things off," said Darrell West, director of governance studies at the Brookings Institution in Washington.
PLANNING TO WATCH
The address to a joint session of Congress will start at 8 p.m. EDT (0000 GMT) and last for about 30 minutes, Gibbs said. Polls say many Americans plan to watch.
Obama discussed healthcare with Senate Majority Leader Harry Reid and House of Representatives Speaker Nancy Pelosi on Tuesday afternoon. The two Democrats expressed optimism after the meeting that a reform measure would pass.
"In our conversations today, we think we're up to 90 percent of things that are agreed upon. We have 10 percent that we have to work on and we can do that," Reid told reporters after the White House meeting.
Legislators have offered a variety of proposals, but appear divided over most of them.
Senate Finance Committee Chairman Max Baucus, a Democrat who leads a group of six senators trying to craft compromise legislation, will put forward a plan including sweeping insurance market changes and a fee on companies that will help pay to cover the uninsured, said a source familiar with the proposal.
It calls for non-profit cooperatives to compete with insurance companies but does not contain a new government-run health insurance plan -- the "public option" -- sought by many liberal Democrats and backed by Obama, the source said.
Pelosi said she believed the House would not pass a reform bill without a public option, which is fiercely opposed by insurance companies and many Republicans.
Olympia Snowe, a Republican member of the Senate negotiating group, supports a compromise that would not initially include a public option but would "trigger" the creation of a government program if insurance companies failed to meet cost and quality benchmarks.
Obama's speech will be aimed at least as much at Democrats in Congress as the public. If he can energize and unite them, and his Democratic base, he can pass healthcare reform, analysts said.
"The game has now come down to: 'Can the administration hold the Democrats in line for a bill that actually amounts to something?'" said James Morone, a Brown University professor and author of "The Heart of Power: Health and Politics in the Oval Office."
"He's got the votes if they've got the heart and courage to stick together," he said. (Editing by Chris Wilson)