While one may pragmatically vote for a flawed candidate, one may not vote for anyone who advocates and enables unmitigatedly evil acts, and that includes abortion. “In the case of an intrinsically unjust law, such as a law permitting abortion or euthanasia, it is therefore never licit to obey it, or to ‘take part in a propaganda campaign in favor of such a law, or vote for it’” (Evangelium Vitae, 73).
At one party’s convention, the name of God was excluded from its platform and a woman who boasted of having aborted her child was applauded. It is a grave sin, requiring sacramental confession and penance, to become an accomplice in objective evil by voting for anyone who encourages it, for that imperils the nation and destroys the soul.
Read more of what Fr. Rutler has to say here: