"He is a Southern religious conservative; he can speak to conservatives in a way the others cannot," Smith said, adding that Thompson has "consistency" in his statements while "Romney has to start every press conference or statement almost with a disclaimer."

"(Thompson's) not going to have to explain things away," Smith said.

The South Carolina primary has been the litmus test for Romney because it would be the first test of whether Christian conservatives in the South would vote for an LDS candidate. Adding Thompson to the ticket may make it a bigger obstacle.

But while Smith thinks Thompson's entrance into the race may affect Romney, he is not convinced Thompson has "staying power" to eventually become the nominee.

"His resume is thinner, and he doesn't have a lot of hands-on political experience," Smith said. Conservatives will give Thompson a fair hearing, though, Smith said, because none of the candidates "are particularly exciting" to them at this point.

How conservative?

But Kirk Jowers, director of the University of Utah's Hinckley Institute of Politics, said Thompson is not the answer for conservatives dissatisfied with their choices.

"Fred Thompson is not that conservative. His voting record is very much similar to McCain's," especially on issues like campaign finance. He is not that (former Arkansas Gov. Mike) Huckabee 'I-don't-believe-in-evolution' kind of candidate," Jowers said.

Voters don't know much about Thompson yet, Jowers said, but once they do, their interest in him as a candidate will wane. "We always like what we can't have. Once we can have it, it's not so great," Jowers said.

Thompson, he said, is "an empty vessel to most people, so they fill him up with whatever they desire in a candidate ... ; (now) all his foibles and his record will be there and people will start falling away from him."

As for the evangelical Christians who want a conservative candidate but aren't comfortable with Romney's membership in the LDS Church, they may turn to Thompson initially, Jowers said, but they will turn away once they review his record.

"Early, I think it has an impact, because I think people will be happy because they have an alternative who is not a Mormon. But ultimately, they have to compare the individuals, and I think at that stage (Romney) will be fine."

Late in the game

The timing is also important. Thompson's formal announcement may come later this summer, but the Democratic and Republican presidential candidates have been raising money and meeting with voters in the early primary states — particularly Iowa, New Hampshire and South Carolina — practically since the 2004 campaign ended.