WASHINGTON - President Barack Obama puts himself squarely into the center of the debate over U.S. healthcare reform, his top domestic policy priority, with a high-stakes address to Congress Wednesday.

Obama wants to overhaul the $2.5 trillion U.S. healthcare system by cutting costs and expanding coverage to the 46 million Americans without health insurance. But his fellow Democrats who control Congress have struggled to craft a reform bill and most Republicans have fought it.

Here are some questions and answers about the nationally televised speech, set for 8 p.m. EDT (0000 GMT).

WHAT IS AT STAKE?

Unlike the sputtering economy and Iraq and Afghan wars, healthcare reform is a challenge that was not inherited from Obama's Republican predecessor, George W. Bush. Obama's ability to push a plan through Congress is seen as a major test of his leadership, less then eight months into his presidency.

Confusion over how reform would work and concern about costs have helped drive down Obama's once-lofty poll numbers. The speech to a rare joint session of the House of Representatives and Senate during evening prime-time television hours ties Obama's image even more closely to the issue.

Obama campaigned as someone who would work with Democrats and Republicans, and has said he wants a plan to pass with support from both parties. Some analysts have said failing to do so would prove bipartisanship is an impossible dream.

The last Democratic president, Bill Clinton, tried to reform healthcare early in his first term. His failure helped cost Democrats their control of Congress in 1994, and some experts have said a failure by Obama could similarly hurt his party in the 2010 midterm election.

WHAT DOES OBAMA NEED TO DO?

Obama must offer specifics about his plan, analysts say, to squash fears opponents have exploited to fan anti-reform sentiments, such as claims the overhaul would lead to government-funded abortions or create bureaucratic death panels to decide who gets care.

He must spell out how he can pay for the nearly $1 trillion plan without boosting the huge U.S. budget deficit or cutting health insurance coverage for those who have it.

He needs to rally and unite his fellow Democrats. While Republicans have been almost unanimous in opposition, Democrats have been divided, with some liberals demanding more sweeping reforms and some conservatives balking at the cost or worrying that the public option was too much government interference in the private insurance industry.

WHAT ABOUT THE PUBLIC OPTION?

Analysts will be listening closely to what Obama says -- or does not -- about the public option, a government-run insurance plan proposed as an alternative to private insurance.

The public option is heavily supported by Obama's liberal base, but strongly opposed by the insurance industry, which has spent millions lobbying against it.

Proposals floated by members of Congress have included various approaches to the public option, including creating one only if insurance companies fail to meet cost and quality benchmarks and others that leave it out altogether.
HOW WILL WE KNOW IF THE SPEECH WAS A SUCCESS?

Some commentators will instantly deem the speech a success or failure, depending on whether or not Obama offers specifics and spells out how the plan will be financed, or whether his remarks electrify his listeners.

But the true test of the speech will come over time. Analysts said one indication will be whether poll numbers show increased support for healthcare reform, and for Obama himself. Another will be whether legislators receive more word from constituents supporting healthcare reform.

The final indication, analysts say, will be whether a meaningful reform plan passes Congress. The White House has said it wants a healthcare overhaul this year.

WHAT ABOUT HEALTHCARE-RELATED COMPANIES?

Health insurers, stinging from a proposal from Senate Finance Committee Chairman Max Baucus that includes a $6 billion annual fee, have so far been the target for much of the health reform debate. Industry representatives and analysts said they will be listening for just how hard Obama pushes for a public insurance plan that would compete in the private marketplace. They are also anticipating details on how he plans to make coverage more affordable.

Pharmaceutical and medical device makers as well as hospitals and other healthcare companies may benefit from more Americans having health insurance coverage. But drugmakers and device companies, which both also face billions of dollars in fees under the Baucus plan, will be bracing for any further comments from Obama that could affect drug prices or reimbursement from the Medicare and Medicaid government health programs for elderly, poor or disabled Americans.

(Additional reporting by Susan Heavey; Editing by Cynthia Osterman and Jackie Frank)