MANCHESTER, N.H. — He may be a weak one, but Mitt Romney is the frontrunner nonetheless. And that means the six other Republicans on stage here Monday night for the first major debate of the 2012 presidential contest will be looking to derail the former Massachusetts governor and establish themselves as the leading alternative to beat President Obama.

By virtue of having run for president in 2008, Romney has the luxury this time of not needing to accomplish what his opponents still must: introducing himself to Republican primary voters and presenting himself as a viable presidential nominee.

That task is just starting for the six other candidates set to appear in Monday’s CNN debate: former Minnesota Gov. Tim Pawlenty; former House speaker Newt Gingrich (R-Ga.); Reps. Michele Bachmann (R-Minn.) and Ron Paul (R-Texas); businessman Herman Cain; and former Sen. Rick Santorum (R-Pa.).

The debate takes place in the crucial early-voting state of New Hampshire, and one of the likely contenders - former Utah governor and ex-U.S. ambassador to China Jon Huntsman (R) won’t be there.

As the shaky frontrunner, Romney needs to keep his message focused on the economy and jobs. His supporters and critics alike agree this is his strongest issue, and Monday’s debate is Romney’s biggest opportunity yet to demonstrate that he has a command on the economy that is unmatched by all the others.

Romney also needs to make the case against President Obama and build confidence within the Republican Party’s base that he would be the strongest GOP candidate to beat Obama on the economy.

Romney began this effort Monday morning, releasing a slick new online video, titled ”Bumps in the Road,” that attacks Obama over the nation’s high unemployment rate. The film ends with the text, “November 6, 2012,” making it clear that Romney’s campaign is taking the long view.

In the debate, look for Romney to try to tightly focus his responses on the comparison between Obama’s economic recordand his own record as governor of Massachusetts and as a business executive and at the 2002 Winter Olympic Games.

But the other candidates are likely to use Romney as a punching bag, attacking him for his Massachusetts health-care law — which is similar to Obama’s federal law and has drawn sharp criticism from conservatives — as well as other issues where Romney differs from others in the field, such as his belief that humans contribute to global warming.

For Romney, the challenge will be to show he can take the punches, avoid getting dragged into a long debate on policy nuances and pivot the conversation back to his central message of jobs and the economy.