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President Obama plans to unveil a $400 billion jobs plan Thursday night, as he attempts to jolt the economy out of a motionless slump and convince skeptical Republicans that his proposals are the right medicine.

Democratic officials familiar with the plan, which will be called The American Jobs Act, confirmed its size to Fox News. The bill is expected to contain a blend of infrastructure spending, tax relief, unemployment assistance and other aid.

Officials said the president will pay for the plan with spending cuts, though he will not detail them Thursday night. However, presidential adviser Valerie Jarrett told Fox Business Network that the bill sent to Congress next week will be paid for through changes in the tax code.

"We need a fair tax system. We need to make sure that people that are paying their fair share, that those who can most afford to pay a little bit more do. ... In this particular bill, the president will be looking to close loopholes."

Obama may ask a new congressional super committee debt panel to go beyond its target of finding $1.5 trillion in deficit reduction by the end of November to cover the cost, allowing the extra savings in the longer run for short-term economic help. That debt panel meets for the first time Thursday before the president's speech.

Republicans have voiced frustration with the whole affair. They expressed concern that the president would use the address to paint Republicans as obstructionists, though they still don't have his plan in hand.

"I can't listen to another political speech from this president," Sen. Jim DeMint, R-S.C., said. "It will be a lecture."

Sen. Kelly Ayotte, R-N.H., told Fox News people are "tired of speeches," but predicted the public would "see right through that" if Obama tries to prop up the GOP to take the fall for the ailing economy.

Democratic officials indeed said Republicans would be sending a signal to the American people that they are on their own if Congress does not pass Obama's legislation. But they described the president's proposals as bipartisan, suggesting the only reason Congress wouldn't pass them is if partisan politics got in the way.

White House Press Secretary Jay Carney called the planks of the bill "common-sense, sensible" ideas.

"Every idea in this ... piece of legislation that we will drop next week in Congress is something that has had traditional bipartisan support," he told Fox News.

Obama is expected to speak for up to 45 minutes, beginning at 7 p.m. ET.

Before Obama even said a word, political and economic reality raised two questions: Will any of his ideas get approved, and will they actually work?

When asked about some of the ideas Obama is expected to discuss, majorities of Republicans, Democrats and independents were all skeptical that the proposals would do a lot to create jobs, a Pew Research Center poll out Wednesday found. A series of new polls by major news organizations finds that the mood of the country is downright dismal about the direction of the country, with Obama's standing and approval on the economy at or near the lowest levels of his presidency.

Democrats familiar with the president's plans say the White House sees the speech as a pivot point after spending the spring and summer focused on negotiations over deficit spending. They say the fall offers the president a window to press congressional Republicans to act on his economic plan -- and if they don't, Obama will spend 2012 running against them as obstructionists. Whether that's enough to win over voters is another matter.

In one upbeat sign for those looking for a Washington compromise, House Speaker John Boehner and House Majority Leader Eric Cantor have told Obama they see potential areas of agreement on jobs -- for example, infrastructure, which Obama has pushed repeatedly. Cantor also signaled to reporters Wednesday that he might support a payroll tax cut.

"It is not games and politics for people out across this country. It's real," Cantor said about the state of the economic debate. "The fact that we have had such sustained joblessness in this country, the fact that people are doing anything they can in many instances just to stay afloat and to pay the bills, it's real."

At the heart of Obama's plan will be extending, by one more year, a payroll tax cut for workers that went into effect this year. The president wants the payroll tax, which raises money for Social Security, to stay at 4.2 percent rather than kick back up to 6.2 percent. That tax applies to earnings up to $106,800.

Obama is expected to seek continued unemployment aid for millions of people receiving extended benefits. That program, too, is set to expire at year's end.

Among the other potential proposals by Obama:

-- Tax credits for employers who hire

-- A major school construction initiative

-- Aid to local governments to prevent layoffs of teachers and other workers

-- Other tax help for businesses, such as continuing to allow them to deduct the full value of new equipment

Since Obama took office in January 2009, nearly 2 million Americans have lost jobs. Almost 14 million people are out of work.

The unemployment rate, which stood at 5 percent at the start of the recession and 7.8 percent when Obama began in office, is at 9.1 percent. After a period of steady if modest job creation, employers have stopped hiring. There was no net change in jobs in August.

Fox News' Ed Henry and The Associated Press contributed to this report.