WASHINGTON -- President Barack Obama steps to the podium in the Capitol Building Tuesday night for his third State of the Union address, with the economy front and center in the public's mind and his popularity at a low ebb.

With a national television audience and a surprisingly volatile presidential primary year for Republicans, experts expect Obama to use the annual speech to boost his campaign to win another four years in office.

"The State of the Union address is a combination of a political speech and a policy speech, because the two things are linked together by our system," said Duke University political scientist David Rohde.

Obama has previewed some of the populist themes he'll touch upon Tuesday night -- notably income inequality and government's role in leveling the playing field for the middle class.

But the effectiveness of the annual address depends on many things, experts said, including delivery, tone and resonance.

Here are five things that veteran politics watchers say Obama must accomplish when he begins speaking at 9 p.m.:


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Stay above the fray

Obama must offer a contrast to the din of harsh campaign ads in the Republican presidential primary, political experts said.

"All he has to do is look presidential," said Hofstra University presidential scholar Meena Bose, despite the attacks by his GOP would-be challengers.

"What Obama as president wants to do is rise above the charges," Bose said. "The president wants to be on the offensive, not on the defensive."


Stress economic gains

The economy may have helped elect Obama three years ago, but it now has become a sore spot -- a target for Republican foes, a source of pessimism for Americans, according to polls, and an issue the president might want to avoid.

But Rohde said, "He has to take it dead on."

White House spokesman Jay Carney said the economy will be central to the speech.

Lynn Vavreck, a UCLA political scientist, said Obama should stress the 3 percent GDP growth in the fourth quarter of 2011 and the half percent drop in the unemployment rate, from 9 percent to 8.5 percent in the past few months.

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"The winning message is not about the level of unemployment," Vavreck said. "It's about the trend line."

Obama needs to follow this line, Rohde said: "It's getting better; it's getting better more quickly recently than before; and the expectation is that it will continue to get better."


Engage Congress

Obama has made no secret of the fact that he intends to run against Congress this year.

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He is expected to include a sharper edge in addressing lawmakers, particularly the House GOP. But he also must show he wants to work with Congress to create jobs.

"The president should call on them to shoulder the burden of leadership and policymaking," Bose said.


Show he's at work

Polls show Americans are fed up with Washington because it can't get anything done. Obama needs to show he is, nonetheless, working hard.

"The overwhelming message of this address is going to be competence and getting things done," Bose said.

"You'll really see the benefits of incumbency," she said, as he talks about steps he'll take without Congress.


Drive home his themes

Obama said he'll lay out an economic blueprint, offering new initiatives on manufacturing jobs, energy and skills training. But he'll also revisit measures Congress has stalled.

"President Obama really needs to show he is in control of the situation, that he has a clear plan and it's up to Congress to respond," Bose said.

After the speech, Obama heads out for an official five-state tour over three days to push his agenda.

Vavreck said the speech should clarify his belief that the economy is on the mend: "Now that things are getting better, don't change horses midstream."