With stock markets crashing, banks reeling and millions of people seeing jobs and savings disappear, Obama is likely to grab his best chance so far to use the crisis as a catalyst to enact a sweeping political agenda.

"I think what you'll hear from the president tonight is a sober assessment about where we are and the challenges that we face," Obama's spokesman Robert Gibbs told ABC in a series of television appearances promoting the speech.

But Obama is also under pressure to rekindle the hopeful rhetoric and optimism which swept him to electoral victory last November in a nation browbeaten by weeks of economic gloom.

"You'll hear the president end tonight with a strong, hopeful message that we've met many of the challenges that have always faced this country and come out on the other side for a brighter America," Gibbs told CNN.

The joint address to Congress, similar to a State of the Union speech, which presidents traditionally give in subsequent years of their administration, is also Obama's first chance for a formal roll out of nuts-and-bolts policy.

He is set to address healthcare, climate change, energy and education, as he argues that the size of the crisis means his key agenda items are more, not less important than they would have been in sunny economic times.

Obama has already driven a 787-billion-dollar stimulus plan through Congress, unveiled a 275-billion-dollar foreclosure plan and tried, so far unsuccessfully, to steady the banking industry.

The president will step up to the lectern in the House at 9:00 pm (0200 GMT) for the most important speech of his presidency so far, riding on a wave of popular support, despite a daily diet of devastating economic news.

Sixty-eight percent of those asked in a Washington Post/ABC News poll out Monday approved of Obama's job performance and 60 percent were happy with the way he is dealing with the economy.

A New York Times/CBS News poll Tuesday showed Obama had a 63 percent approval rating, and more than 75 percent said they were optimistic about the next four years with him as president.

A CNN/Opinion Research poll revealed that a solid majority -- 67 percent -- feel that the policies Obama is proposing is moving the country in the right direction.

Overseas, US foes like Iran and North Korea will sift Obama's words for diplomatic signals, while US allies will follow emerging policy in Iraq, Afghanistan, the Middle East and Asia.

After vowing to cut a gaping budget deficit he inherited from the Bush administration in half by 2013, the president will also likely seek political room for maneuver required to rein in spending.

Obama will also likely talk of bridging the partisan divide, even though few Republican legislators voted for his economic stimulus plan.

One leading Republican, Congressman Darrell Issa, told reporters at a White House "Fiscal Responsibility Summit" on Monday he was eagerly awaiting the speech.

"You never know what the salesmen is going to sell you until he turns up at the door," he said, but warned Obama to avoid a "laundry list" of spending plans.

Rising Republican star Bobby Jindal, the Indian-American governor of Louisiana, and a possible future presidential candidate, will deliver the Republican response to Tuesday's presidential address.

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