A number of years ago, William F. Buckley, Jr. was discussing the abortion issue with a guest on his program, "Firing Line." The guest challenged Buckley's assertion that a conservative should not vote for a candidate who supported abortion. The guest charged that this was "single issue" voting.
Not at all, said Buckley. "Abortion is not a single issue," he explained: "It is a disqualifying issue."
If you had, say, a candidate with whom you agreed about everything, but he was in favor of bringing back slavery, Buckley asked, would you vote for him? No. Because slavery is a disqualifying issue.
Abortion operates in the same way (or should) for a voter who believes that life begins at conception. As Buckley explained, a prolife voter should no more vote for a pro-abortion candidate than he would for a person who favored infanticide--because that's exactly what abortion is.