Here's what has people concerned: Thompson's 1994 Issue Positions
Here's the source as described in Ryan Sager's blog article (linked above):
Project Vote Smart, which compiles voting records and other background materials on politicians, has finally put up its page on Fred Thompson (OK, maybe "finally" isn't exactly the fairest word when Mr. Thompson hasn't even announced for president — but I've been eager to see it).
Anyway, it seems Mr. Thompson filled out a survey for Project Vote Smart back in 1994, when he was running for Senate. While it's mostly pretty predictable (boo foreign aid, yay low taxes), there are a few parts worth scrutinizing... (see: abortion, education, AIDS)
Here's the part about abortion:
Under abortion: He checked the box for: "Abortions should be legal in all circumstances as long as the procedure is completed within the first trimester of the pregnancy." He did, however, support a number of restrictions on abortion: requiring parental notification, allowing states to impose waiting periods, and eliminating all federal funding of abortion. Lastly, he said Congress should leave legislation on abortion to the states.
Fred Thompson has a solid pro-life voting record, but he has gotten more pro-life over time. Of all of the viable presidential candidates (and although he hasn't announced, he will almost certainly run) Fred Thompson has the most solid pro-life voting record, and he seems to be the most solidly pro-life candidate.
Other positives about Fred: he's against gay marriage and he's not a gun grabber.
He says he is solidly pro-life now and that his convictions have solidified since the birth of his daughter.
See also: Thompson To-Dos
Here's the part about abortion:
Third, acknowledge that you've gotten more pro-life over time. Twice in recent weeks, you have expressed perplexity that anyone thinks you were once pro-choice. Stephen Hayes quoted you in The Weekly Standard:
"I have read these accounts and tried to think back 13 years ago as to what may have given rise to them. Although I don't remember it, I must have said something to someone as I was getting my campaign started that led to a story. Apparently, another story was based upon that story, and then another was based upon that, concluding I was pro-choice."
But, he adds: "I was interviewed and rated pro-life by the National Right to Life folks in 1994, and I had a 100 percent voting record on abortion issues while in the Senate."
Your record in the mid-1990s was a bit less solidly pro-life than that. A 1994 issue of Republican Liberty apparently quotes you opposing public financing of abortion but adding: "The ultimate decision must be made by the woman. Government should treat its citizens as adults capable of making moral decisions on their own." That same year, in which you ran for the Senate (and won), you said something similar in a debate: There should be no federal funding, and states should be allowed to enact parental notification and other "reasonable controls," but government should not "come in and criminalize, let's say, a young girl and her parents and her doctor as aiders and abettors that would be involved."
News accounts treated you as pro-choice, and there is no record of your campaign's trying to dispute that characterization. The National Right to Life Committee did indeed endorse you in that race, and their post-election newsletter listed you among the victorious "pro-life candidates" that year. But that newsletter also grouped you with candidates who were opposed to the Freedom of Choice Act and federal funding of abortion, rather than with candidates who were pro-life across the board.
In 1997, finally, your office sent a constituent a letter about abortion that included this line: "I believe that government should not interfere with individual convictions and actions in this area."
I think the record suggests that you were always uncomfortable with abortion and prepared to support some restrictions on it, but that your opposition deepened over the course of your time in public life. The whole country's discomfort with abortion seems to have deepened over that time, too. (In part, that was a result of the partial-birth abortion debate in which you were involved.) If that is what happened, I don't think pro-lifers will hold it against you to say so. Those pro-lifers who worry about the sincerity of Mitt Romney's conversion do so because he seemed ardently pro-choice not long ago. As you said, you have a strong record of voting with pro-lifers that goes back to 1995.
That makes sense, and Fred Thompson's 100% pro-life voting record, as well as his repeated statements professing his solid, pro-life position as he prepares to run for the presidency should help people see that he's grown over the years, is now more pro-life than he ever was (even when he was voting pro-life as a senator), and can be trusted. He has said he thinks Roe vs. Wade was a mistake, that he supports overturning it, and that he is opposed to judges legislating from the bench.
I don't see any difficulty with continuing to support Fred Thompson.