It is fair to say that Fred Thompson tried to straddle the abortion issue, just like Mitt Romney. He described himself as pro-choice while consistently voting for abortion restrictions. When the pro-life movement largely abandoned its push for a constitutional amendment and shifted to a more incremental strategy, it became possible for politicians to vote the pro-life line in Congress while still maintaining support for legal first-trimester abortions and, in some cases, Roe. This was George Allen’s position, for instance.
There are two differences between Thompson and Romney, however. Thompson never took a position as close to the NARAL line as Romney did in 1994 or 2002. Second, the former Tennessee senator has compiled a longer record of being operationally pro-life even if nominally pro-choice. Thompson’s National Right to Life Committee scorecards, which I can’t find online, show disagreements over campaign finance reform but not abortion or cloning.