UPDATE 1-Romney Vs Perry Rivalry in US Debate Spotlight

* More fireworks possible from Perry, Romney

* Tea Party members will ask questions of candidates

* Perry calls for 'frank discussion' on Social Security

(Adds Perry quotes, Pawlenty endorses Romney)

By John Whitesides

TAMPA, Fla. (Reuters) - The budding rivalry of Republican presidential contenders Rick Perry and Mitt Romney takes the spotlight Monday when the party's White House hopefuls meet in Florida for their second debate in less than a week.

The debate, co-sponsored by the Tea Party Express advocacy group, will include questions from members of the conservative Tea Party movement that has reshaped the U.S. political scene with its focus on limited government and reduced spending.

It will focus heavily on the economy and is likely to feature more fireworks between Perry and Romney, who had several sharp exchanges last week over the Social Security retirement plan and job creation.

Perry, a conservative Tea Party favorite, has zoomed past Romney to lead in opinion polls since entering the race last month for the Republican nomination to challenge President Barack Obama in 2012.

But Romney got a boost on Monday with the endorsement of Tim Pawlenty, the former governor of Minnesota who dropped out of the race after a poor showing in last month's Iowa straw poll, an early test of campaign strength.

Like Romney, Pawlenty based his campaign on his ability to appeal to the more moderate voters needed to win a general election. Perry's run to the top of Republican polls has been powered by his strength with social and religious conservatives.

The debate, which begins at 8 p.m. EDT and will be telecast on CNN, will be held in Tampa, Florida, the site of the party's nominating convention in less than a year.

Perry, the governor of Texas, called Social Security a "Ponzi scheme" and a "monstrous lie" during last week's debate. That drew a rebuke from Romney, the former governor of Massachusetts, who said he supported the popular retirement program but wanted to fix it.

In a column in the newspaper USA Today, Perry said on Monday the goal should be fixing Social Security by making it more financially sustainable for the long-term.

"We must have a frank, honest national conversation about fixing Social Security to protect benefits for those at or near retirement while keeping faith with younger generations who are being asked to pay," he said.

Perry's debate comments on the subject will be watched closely in Florida, which has the country's biggest proportion of elderly voters and hosts a potentially vital nominating contest next year.

"It's going to be a good test of Perry's ability and his staying power. He's going to be on the spot again," said Steve Roberts, a former state party chairman and Republican National Committee member.

Among the other candidates vying for attention on the crowded Florida stage will be U.S. Representative Michele Bachmann, who has seen her campaign begin to fade since winning the Iowa straw poll last month.

Perry has robbed Bachmann of much of her support from conservatives, which is crucial in states with early nominating contests like Iowa and South Carolina.

Other candidates participating will be Jon Huntsman, the former Utah governor, former U.S. House of Representatives Speaker Newt Gingrich, businessman Herman Cain, U.S. Representative Ron Paul and former U.S. Senator Rick Santorum.

The debate will be the fifth so far of the Republican presidential campaign. It will be followed closely next week by another debate in Orlando, Florida, as the White House race heats up. (Editing by Christopher Wilson)