WASHINGTON — Barack Obama is cruising into the presidential debates with momentum on his side, yet he’s still struggling to revive the passion and excitement that propelled him to the White House. Mitt Romney is grasping for his last, best chance to reboot his campaign after a disastrous September.

The fierce and determined competitors in the tight race have a specific mission for the three debates, the first of which is Wednesday night in Denver.

Obama, no longer the fresh face of 2008, must convince skeptical Americans that he can accomplish in a second term what he couldn’t in his first, restoring the economy to full health.

Romney, anxious to keep the race from slipping away, needs to instill confidence that he is a credible and trusted alternative to the president, with a better plan for strengthening the economy.

“The burden in many ways is heavier on Romney,” says Wayne Fields, a professor at Washington University in St. Louis who specializes in political rhetoric. “What we see right now is an uncertainty about whether he’s ready for the job.”

For all the hundreds of campaign appearances, thousands of political ads and billions of dollars invested in the race, this is a singular moment in the contest. Upward of 50 million people are expected to watch each of the debates, drawing the largest political audience of the year.

Forty-one percent of Americans reported watching all of the 2008 debates, and 80 percent said they saw at least a bit, according to a Pew Research Center poll.

That intense interest tends to crowd out everything else for a time, adding to the debates’ importance. With polls indicating that Obama has been gaining ground steadily in the most competitive states, the pressure is on Romney to turn in a breakout performance.

The Denver debate, 90 minutes devoted to domestic policy, airs live at 9 p.m., with the two men seated side by side in elevated director’s chairs. Romney and Obama debate again Oct. 16 in Hempstead, N.Y., and Oct. 22 in Boca Raton, Fla. Vice President Joe Biden and Republican Paul Ryan have their lone debate Oct. 11 in Danville, Ky.

With early or absentee voting already under way in more than half the states, any first impressions created in the debates could well be last impressions. What the candidates say is sure to matter immensely, but how they say it may count for even more.

“We remember visual impressions from debates more than we remember specific words,” says Alan Schroeder, a Northeastern University professor who’s written a history of presidential debates.

Whether the candidates smile or grimace, strike a confident or defensive pose, speak with a resonant or strained tone of voice, it all matters. That may be particularly true for the all-important undecided voters and those still open to changing their minds.

Staunch Democrats and Republicans may well be firm in their choices, says Patti Wood, an Atlanta-based expert on body language, but if less partisan voters are “frightened in general about their lives, if they’re insecure, they’re going to pick the most charismatic person.”

Both candidates have challenges to overcome on that score, according to Wood.

Obama, 51, has been sounding “very tired and very strained” lately, she says, and Romney, 65, “has a problem with appearing superior and cold.”

Overall, she says, “Romney is looking a little bit younger than Obama right now,” in terms of energy if not wrinkles.

Both candidates are experienced and competent debaters. But each, setting the judgment bar high for his opponent, is working overtime to puff up the skills of the other guy and play down his own debate credentials.

Romney recently described the president as “eloquent in describing his vision” during the 2008 debates. But the GOP nominee added that Obama “can’t win by his words, because his record speaks so loudly in our ears.”

Obama campaign spokeswoman Jen Psaki stresses that Romney has been preparing for the debates with “more focus than any presidential candidate in modern history.” Sketching sky-high stakes, Psaki says the Republicans fully expect the debates to be “their turning point” in the campaign.

The president himself mocked the idea that Romney still can alter the campaign dynamic.

“Every few days he keeps on saying he’s going to reboot this campaign and they’re going to start explaining very specifically how this plan is going to work — and then they don’t,” he said last week while campaigning in Virginia.

For all their positioning, both candidates will use the debates to try to surmount the same challenges that they long have confronted.

Romney, frequently criticized for shifting his positions to sync up with the politics of the moment, needs to project “a kind of character, a kind of maturity that allows him to be presidential,” says Fields.

Obama, an incumbent who’s shown himself to be comfortable in the media glare, “doesn’t have to prove that part,” says Fields. “He has to prove that he has real answers to problems that have not been solved in his first term and for which there is a great deal of unrest.”

Romney is sure to be questioned anew about his caught-on-video comment dismissing the 47 percent of Americans who don’t pay federal income tax as victims who won’t take responsibility for their lives.

“How can Gov. Romney have such a profound misunderstanding of the people of this country?” Biden asked during an appearance Saturday in Fort Myers, Fla., “When I hear this talk, not just from Romney and Ryan, but from this new Republican Party … I don’t recognize the country they’re talking about.”

Former President Bill Clinton, offering a bit of unsolicited advice to the opposition, says Romney would be wise not to “double down on that 47 percent remark.”

“That will cause difficulties, because we now know that the overwhelming number of those people work and have children,” Clinton said recently. He added that the most important job for Romney is to “find a way to relate to more people in these debates and speak to more of them.”

On Saturday, the Obama campaign posted a Web video urging debate viewers take Romney’s claims of private-sector experience with a grain of salt. “Remember, it wasn’t about creating jobs,” the video says. It includes testimony from steel- and paper-plant workers laid off after Bain Capital takeovers.

Also Saturday, the Romney campaign announced plans for his wife, Ann, to speak at a rally Monday in Henderson, Nev., where Obama is planning three days of private debate preparation. And Romney points to Syria, Libya and Iran to criticize Obama’s foreign policy as “one of passivity and denial” in his weekly podcast.

Meantime, there’s no shortage of advice swirling around the two candidates: loosen up, study up, be aggressive, don’t overdo it, admit mistakes, don’t apologize, project confidence, ooze emotion, use humor, make eye contact, get more sleep.

It’s enough to paralyze even the most skilled orator if not kept in perspective.

“That’s what so tricky about this,” says Schroeder. “Debates themselves are this kind of interesting blend of the choreographed and the spontaneous. … What you want is for the candidate to be prepared but not to overlook those opportunities to improvise when you see an opening.”

The stakes are lower for the debate between Biden and Ryan. It offers the prospect of a looser and more entertaining discussion between two candidates with vastly different styles and personalities.

In 2008, Biden’s debate with Republican Sarah Palin attracted 70 million viewers, easily topping the 63 million high-water mark for the presidential debates that year.