Now, as CBS News' John Bentley reports, his poll numbers are slipping as his campaign scrambles to stay in the race.
What happened? Can Thompson turn it around?
Asked about his numbers, Fred Thompson often mentions former Democratic presidential candidate Howard Dean.
"President Howard Dean was leading in all the polls, was spending most of the money, and he got anointed by everybody. On election night, ol' Howard wasn't anywhere to be seen," said Thompson. "And Kerry, who, you know, had been in single digits, wound up winning it. So the only thing that we know is that no one knows nothin' when it comes to politics."
But according to University of Miami political science professor Greg Koger, the problem may be deeper than just a recent drop in the polls.
"It's a puzzling candidacy," Koger said. "I mean, on paper, he could be a very good candidate for the Republican Party. He has a set of views that are the type of positions that can unite the Republican Party. On the other hand, the fact that he got in late is really hurting his ability to campaign."
There have been some sparsely attended events, like one in New Orleans, for example, where barely a third of the room was filled. But Thompson has also won accolades for his policies on national defense, Social Security reform, and a revamped tax code.
So, why haven't those ideas resonated with voters?
"When you're going to come up with a military paper, with a tax paper, with a Social Security paper, and all those things, it takes some work to get all those things done, to sit down with the experts and actually work through those," said Rich Galen, a Thompson advisor. "So while those weren't public campaign events, they were certainly campaign activities."
"Now we're into the public campaign. It's back to the lunch rooms, the luncheonettes, the dining rooms. All those kinds of things where everybody will travel around in buses and SUVs. I think that's what you're seeing now, that Senator Thompson is at that phase of the campaign," Galen said.
You will also see Thompson spending more time appealing to religious voters.
Thompson joked about that while speaking at a Miami church on a recent Sunday morning.
"My mother saw to it I was in church three times a week, but she would be very surprised that I'm on this side of the pulpit," he said.
His appearance served a dual purpose for the campaign: shore up his religious credentials after he said he didn't attend church regularly, and appeal to conservative voters who may have abandoned him for Mike Huckabee, a Baptist minister.
A popular theory in political circles is that voters dated Fred Thompson, but married.
"It's a pretty good line. My sense is though, that based upon the scrutiny Governor Huckabee's coming under now that he's the front-runner, the marriage may well end in Reno with a quickie divorce," said Galen.
At the very least, voters are having commitment issues. A recent CBS News poll says over three-quarters of Republican primary voters haven't made up their minds yet.
"We're tied for third in Iowa," said Galen. "We're in second place, or tied for second, in South Carolina. We're not doing that well in New Hampshire. We're second, or tied for second, or maybe third, in the national polls… He's in the thick of the first-tier candidates."
That's why Thompson is now spending virtually all of his time in the vitally important state of Iowa.
Iowa volunteer Richard Rogers said "Thompson hasn't had a chance to be here as much as many of the others," but that "he's planning to be here almost full-time until the caucuses. I believe that it can make difference, even this late."
Professor Koger is not as optimistic.
"The best thing would be to build a time machine and go back to 2006 and start this early. Meet people within the Republican Party early, get them excited about his campaign, and make the case that the party ought to unite behind Thompson," Koger said. "I think it's difficult to make up ground in middle 2007, late 2007… I just think that's going to be very difficult."