DENVER -- President Barack Obama and Republican challenger Mitt Romney — who has struggled to find momentum — will offer voters two starkly different prescriptions for fixing the ailing economy as they duel Wednesday in their first and perhaps most critical debate.

More than 60 million people are expected to watch when the nationally televised, 90-minute debate kicks off at 9 p.m. EDT, far more than watched the two major party national conventions and dwarfing the number that watched Romney in Republican primary debates.

Underscoring the significance, the men will arrive at the University of Denver debate site after days of closed-door rehearsals: Obama in Nevada and Romney in Colorado. The stakes are particularly high for Romney, the former Massachusetts governor who has stayed close to Obama in most polls but continues to trail.

“It’s one of the few possible game changers left for him, and the only one he has a certain amount of control over,” said Tobe Berkovitz, an associate professor of advertising at Boston University.

Most polls show Obama remains vulnerable — his Gallup job approval rating Sept. 28-30 was 47 percent, about where it’s been for some time, and a Quinnipiac Polling Institute survey released Tuesday, taken Sept. 25-30, put him ahead of Romney 49 percent to 45 percent.

Obama also battles high expectations. Romney has not engaged in a one-on-one political debate since he ran for governor of Massachusetts 10 years ago, while Obama debated Republican John McCain three times in 2008 and is a familiar presence on American television.

Regardless of political spin from the campaigns, Americans by a 2-to-1 margin expect Obama to win the debate, according to polls.

The numbers suggest an opportunity for Romney, who will try to tell voters that Obama should be held responsible for a stubbornly sluggish economy. Romney plans to stress that Obama’s remedies too often involve “going forward with a stagnant, government-centered economy,” said senior advisor Ed Gillespie.

Obama is trying to lower those expectations. “The president is familiar with his own loquaciousness and his tendency to give long, substantive answers,” said Obama campaign spokeswoman Jen Psaki, calling it a challenge when there’s a timer running. “That’s certainly something he and all of us are cognizant of.”

“Gov. Romney, he’s a good debater. I’m just OK,” Obama said with a grin to a Nevada audience earlier this week. “But what I’m most concerned about is having a serious discussion about what we need to do to keep the country growing and restore security for hardworking Americans. That’s what people are going to be listening for.”

Alan Schroeder, a Northeastern University professor who has written a history of presidential debates, Presidential Debates: 50 Years of High-Risk TV, said Obama in 2008 tended to be “very agreeable” with McCain.

“That was part of his strategy, to appear willing to extend the olive branch, but I don’t think that’s going to fly this time,” Schroeder said.

Barring a major gaffe or surprise, few analysts expect the debate to radically change the race right away. Quinnipiac found that 86 percent thought it would make no difference in how they voted.