WASHINGTON — He has chosen the most dramatic forum a president commands to answer a question looming over Washington: Who runs the town?

Apart from reviving his health care plan, President Obama needs the prime-time address today before a joint session of Congress to reassert his grip on a political apparatus that will soon determine whether his agenda succeeds or fails.

A summer of setbacks for Obama has left him in a weakened position. Once the dominant communicator in American politics, he has seen the health care debate sidetracked by false warnings that government "death panels" would be employed to snuff out grandma. Distractions arose over past remarks made by midlevel aides. Even a benign back-to-school speech Obama gave to students yesterday became a vehicle for conservative activists to warn of presidential "indoctrination."

Obama's poll numbers slipped during the summer break, but more worrisome for the White House is that power shifted to Congress as it grabbed control of the details of the health care overhaul that was a centerpiece of Obama's 2008 campaign.

Whether Obama succeeds in overhauling health care could shape the rest of his presidency, said Robert Reich, a former labor secretary in the Clinton administration.

"He has huge fights ahead with lobbyists and special interests," Reich said. "They're all watching. They're all waiting to see how he manages. It will set the direction.

"The weeks ahead will either enhance his power and authority or it may diminish it."

In the Obama arsenal, a speech is often the preferred method of coping with political crises. White House aides said Obama would use the appearance today to specify what he wants in a health care bill and revive its prospects.

That will require some deft maneuvering. Obama must find the right mix of provisions to draw in conservative Democrats and perhaps a few Republicans without alienating liberal lawmakers.

TOUGH CHALLENGES

Obama's speech will also provide a chance for him to re-establish his presidency as what John F. Kennedy called the "vital center of action" in the government.

He will need all the political clout he can muster. Beyond winning the health care battle, Obama is facing a deteriorating military landscape in Afghanistan. This year has been the most lethal for American forces, with 190 deaths. Obama is also confronting tough diplomatic challenges in the Middle East and Iran.

On the economic front, unemployment is closing in on 10 percent and is expected to exceed that threshold in the coming months.

Wading into these fights, Obama will be better positioned if he prevails in the health care debate, liberal supporters said.

Robert Borosage, co-director of the liberal Campaign for America's Future, said the health care bill "is the prism by which all of the achievements to date will be seen. If it looks like he's rolled on this or frustrated on this, it will hurt significantly across the board and on the rest of his agenda."

POLITICAL FUROR

For much of the summer, Obama was forced into a reactive role as the political climate became highly polarized.

On Sunday, environmental adviser Van Jones resigned. He had been targeted by conservative talk show hosts, including Glenn Beck and Sean Hannity, as a "radical" associate of Obama. Jones came under sharp criticism for coarse comments he had made about Republicans and for signing a petition questioning whether the U.S. government had a hand in the attacks of Sept. 11.

Another target for conservatives is Cass Sunstein, Obama's nominee to be White House regulatory chief. A constitutional law professor, Sunstein is coming under attack for his writings on guns and animal rights. In a 2002 essay, he suggested banning hunting when the sole purpose is "human recreation."

On the eve of an expected vote in the Senate to confirm Sunstein, several hunting groups have mobilized to defeat his nomination.

An Obama administration spokesman, Kenneth Baer, said in response: "Cass has been very clear in his writing as well as in his testimony to the Senate Governmental Affairs committee that he is a strong believer in the Second Amendment."