Fred Thompson said Monday he is "100 percent pro-life," despite earlier statements that he would not support a constitutional ban on abortion.

Speaking to FOX News, the former Tennessee senator expanded and revised a statement he made on NBC's "Meet the Press" Sunday that he would not support a constitutional ban on abortion, and that such decisions should be left to the states.

After talking about the importance of border security, fiscal responsibility and restoring credibility to Washington at the "Politics and Eggs" breakfast in Bedford, N.H., a ritual in the early-voting Granite State, Thompson pledged always to support efforts to restrict abortion rights.

"My record is clear. One hundred percent pro-life voting record over eight years at the national level. The proof is in the pudding there. Go back and look at all that record. My whole career I have said Roe versus Wade was wrongfully decided. I've been pro-life all my career and always will be," Thompson told FOX News, alluding to other Republican candidates who have refined their positions since declaring their presidential ambitions.

"The difference with me and most of the rest of them is that's where I was yesterday, that's where I am today and that's where I will be tomorrow," he said.

Thompson also returned volley on a sharp hit by another GOP presidential candidate, Mike Huckabee, who criticized Thompson's performance Sunday as weak on conservative issues.

"Thompson's comments ... were disappointing and disheartening for those who were expecting him to be a solid voice for conservatives. The marriage and life amendments are critical issues for those of us who have been on the front lines of these cultural battles. Sen. Thompson's philosophy seems to be more 'cut and run' when it comes to these issues, rather than stand and lead," said Huckabee, the former Arkansas governor.

Asked about the charge, Thompson said he has a constitutional amendment on same-sex marriage that he thinks will pass Congress as well as any judicial test. He added that Huckabee is harping on Thompson's record because he can't bear scrutiny of his own.

"Governor Huckabee talks about this, I suppose, because it is the only conservative position he has got. People talk about sanctuary cities, he apparently wanted a sanctuary state in Arkansas. He is very weak on immigration policy. He was one of the highest taxing governors that we had in this country ... so I can understand why he might want to talk solely about this issue."

Amid questions about his conservative colors, Thompson also took a step toward solidifying his credentials, announcing that Shannon Royce, former executive director of the conservative Arlington Group, would be joining the campaign as grassroots and special projects director.

Thompson entered the presidential race in early September, and has visited New Hampshire four times. Asked if he hasn't invested enough time in the state, he said he would be returning "early and often."

"There is a lot of ground to cover, no question about it," he said. "We have covered some other ground before we have here, but at the end of the day we will have been here a sufficient amount of time I am sure."

FOX News' Carl Cameron and Cristina Corbin contributed to this report.