(Reuters) - Republican presidential candidates debate in Florida on Thursday as the party's voters consider who they want as their nominee to challenge U.S. President Barack Obama in the November 2012 election.

The following are five things to look for at the two-hour debate, which is sponsored by Google, Fox News and the Florida Republican Party.

DOES PERRY SOFTEN STANCE ON ISSUES IMPORTANT TO ELDERLY?

Florida Republicans will be watching to see whether front-runner Rick Perry, the conservative governor of Texas, modifies his stance on Social Security to make him more palatable to mainstream voters and the swing state's large elderly population. Perry's denunciation of the popular government program for retirees was welcomed by conservatives but risks scaring seniors. While Perry tops Republican public opinion polls, his lead is anything but secure.

CAN ROMNEY RAISE DOUBTS ABOUT PERRY'S RECORD, POSITIONS?

Perry's main challenger, Mitt Romney, is second in the polls after spending most of the year in the lead. His goal is to raise doubts about Perry by focusing on the Texan's Social Security position and other issues that might undercut his lead, including support for a Texas law that granted education benefits to the children of illegal immigrants. But there is a danger in being too negative and Romney will need to temper his assault on Perry by promoting his ideas to improve the economy.

WILL BACHMANN PLAY THE HEAVY FOR ROMNEY?

Michele Bachmann has to elbow her way into what is chiefly a two-man contest for the nomination. The Minnesota congresswoman, the favorite of the low-tax, limited government Tea Party movement until Perry jumped into the race scored important points in a debate last week by criticizing Perry over a vaccination program for adolescent girls in Texas. But her comment that the "government injection" was "potentially dangerous" was widely derided by medical experts. Romney could benefit if Bachmann attacks Perry again, allowing the former Massachusetts governor to focus instead on his own record and proposals.

CAN HUNTSMAN GAIN MOMENTUM?

Former U.S. Ambassador to China Jon Huntsman needs a breakthrough evening. More moderate than the others in the field, Huntsman has failed to catch fire and needs some buzz to keep his candidacy alive into the early voting states next year. A new poll in New Hampshire showed him gaining ground in that important state, but he will need to show broader support to remain viable.

WILL PAUL CONTINUE TO DRIVE THE AGENDA?

Texas Congressman Ron Paul is seen as having next to no chance of winning the nomination, but he consistently gets about 10 percent in Republican polls, giving him an important voice. A libertarian, Paul has helped drive the debate on foreign policy and government intervention in the economy.

(Reporting by Steve Holland; Editing by Peter Cooney)