A quid pro quo? Possibly. Myth Romney made much political hay out if it during the AZ debate at much chagrin for Santo.
Romney: Santorum backed pro-choice Specter in 1996
"Michigan -- Mitt Romney, clearly pleased with both his performance and Rick Santorum's in the Arizona debate, attacked Santorum's pro-life credentials, saying that Santorum has a history of supporting pro-choice Arlen Specter that long predates the 2004 Senate race.
"Now at the last debate -- and that was a fun debate, I've got to tell you," Romney said today, before quoting Santorum's explanation that he "[took] one for the team" when voting for No Child Left Behind.
Romney seemed determined to undermine Santorum's credibility among his core pro-life supporters, although he also criticized the senator on other votes. "There was also in 1996 when he supported Arlen Specter . . . He supported the pro-choice candidate, Arlen Specter," Romney said, against a pro-life candidate, Bob Dole. "This taking one for the team, that's business as usual in Washington." "
-end snip-
Taking one for the GOP team is best answer we will get from him.