WASHINGTON — Barack Obama is cruising into the presidential debates with momentum on his side, but is struggling to revive the passion that propelled him into office four years ago. Republican nominee Mitt Romney is grasping 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 race will be decided in a handful of battleground states which are neither reliably Democratic nor Republican. The US president is chosen in state-by-state contests rather than by national popular vote The Denver debate, 90 minutes devoted to domestic policy, airs live at 9 P.M. EDT (0100 GMT), with the two men seated side by side in elevated director's chairs. Romney and Obama debate again Oct. 16 in Hempstead, New York, and Oct. 22 in Boca Raton, Florida. Vice President Joe Biden and Romney's running mate Paul Ryan have their lone debate Oct. 11 in Danville, Kentucky. 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. 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 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 Republican 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. — AP