No pro choicer has ever won the GOP nod. That sitting senator thing is a moot point, if the other opponent is a senator than someone wins.
REmember, of the senators who ran, all were clearly poor candidates. Goldwater was a good candidate, but he was never going to win against LBJ given JFK's assasination. Mcgovern was a complete leftist kook. Bob Dole was a tired old rino who compromised on everything and wanted to get along too much with democrats. John Kerry was a horrible candidate who flip flopped left and right.
George Allen is a strong candidate who can win, the others could not.