SARASOTA, Fla. — Newt Gingrich has staked his presidential bid on one idea: He is best positioned to defeat President Barack Obama.

Even some of his supporters seem to be struggling to buy that argument, an indication that chief rival Mitt Romney's efforts to undercut Gingrich may be working.

Interviews with more than a dozen Republican voters at Gingrich's overflowing rallies this week suggest that many Florida voters love his brash style as they look for someone to take it to Obama. At the same time, they have lingering doubts about whether the Republican's bomb-throwing alone will make him the strongest Obama opponent.

Romney and his allies have been working hard to stoke those doubts with Florida Republicans ahead of Tuesday's primary. And the GOP's establishment wing has joined in.