An indirect clue to this can be seen in the "evolution" of Democrats' views on abortion as their careers progress. No pro-life Democrat can compete, for example, in Democratic primaries. Thus we see the change come to politicians like Al Gore as they step onto the national stage. In this primary season alone we have as many as three candidates with "evolved" positions. Even this kissy-face article on Kucinich mentions the fact that he went from pro-life to pro-choice in a bootlegger turn. Dick Gephardt, too, talks of his "journey" on the issue. Joe Lieberman is said to have made pro-life pledges in Connecticut, although Lieberman disputes the matter.
On the other hand, David Corn wrote a good, convincing piece for the Nation arguing that Bush is guilty of the reverse flip-flop -- pro-choice to pro-life. And McCain got in some minor trouble in the 2000 primary for dropping pro-choice hints. Both parties are guilty of the same sins of orthodoxy, perhaps. But I'll offer two guesses on the issue. First, a vice-president will have to break with the orthodoxy before a president can. Second, the GOP will do it first. After all, they flirted with it by short-listing pro-choice Tom Ridge for the VP slot in 2000. The pro-life crowd has an influence in Republican politics, but it's not nearly the clout that the groups like NARAL and EMILY's List have on the other side.