Story Highlights• Former GOP Sen. Fred Thompson was the choice of 12 percent in a recent poll
• Thompson trails John McCain and Rudy Giuliani in the USA Today/Gallup poll
• Thompson's star rising because conservatives still looking for a candidate to back
From Mary Snow
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NEW YORK (CNN) -- He's not even officially running for president, but Fred Thompson, star of NBC's "Law and Order" and a former Tennessee senator, is gaining in the polls among Republican White House hopefuls.
This news comes as some of the GOP hopefuls who are already in the race recently lost some ground. What's behind the numbers, and does this mean that Republicans are hungry for a candidate who's not already running?
Roughly two weeks ago, Thompson said he was considering a run for president. Since then, he has skyrocketed out of nowhere to rank third among GOP White House hopefuls in a new USA Today/Gallup poll published Tuesday.
The poll shows former New York Mayor Rudy Giuliani with 31 percent, Sen. John McCain, R-Arizona, with 22 percent, and Thompson with 12 percent of the vote.
He's even surpassed former Massachusetts Gov. Mitt Romney, who was the favorite of only 3 percent in the poll.
Like Giuliani, Thompson can credit fame for his popularity among voters.
"They're both well known. They both have a lot of name recognition. So it appears that some of Giuliani's support, which may have been based on name recognition, is going to Fred Thompson," CNN Senior Political Analyst Bill Schneider said.
But is there something else propelling Thompson's star forward? "One way to read these numbers is, there are a lot of Republicans still shopping around for a candidate," CNN Polling Director Keating Holland said.
Republican strategist David Winston, who is not affiliated with any campaigns, says a key title has eluded candidates Giuliani, McCain and Romney. "What's not occurred, is someone has not grabbed sort of the conservative mantelpiece, the person identified as the lead conservative candidate," Winston said. "And that's been sort of bouncing around with a lot of the candidates jockeying for that, with no one succeeding."
Why hasn't a single candidate pulled ahead with conservatives? Strategists say McCain hasn't gained overall momentum, Giuliani supports abortion and gay rights, and although Romney is now viewed as a conservative, that wasn't always the case.
"The one thing that this poll certainly suggests is that with Fred Thompson getting in the race, the candidate that gets hit the hardest is Romney, " Winston said.
Many now see an opening for Thompson among Republicans. Thompson calls himself "pro-life" and opposes same-sex marriage.
The USA Today/Gallup poll of 1,007 adults was conducted by telephone Friday through Sunday. It has a margin of error of plus or minus 3 percentage points.