We all think in the United States that voting is some sacred practice. This is natural I suppose because it is very central to our culture and system of government. The fact is thought, casting a vote is just our way of influencing our leaders without resorting to violence. Voting for someone because they are Pro-Abortion would certainly be a sin. And voting for someone who is Pro-Abortion because on other important issues they are correct, could also be a sin. For what other issue can possibly compare to 4000 human deaths each day? That trumps the war in Iraq, Vietnam, Korea, heck every act of war the US has been engaged in since WWII in a matter of a few weeks!
But that being said, if you have no choice but to vote for someone who is Pro-Abortion or not to vote at all, I see nothing wrong with voting for the least evil option on the ballot. Casting your vote in this situation does nothing to support their Pro-Abortion stance. Not casting a vote in protest, takes your vote out of the pool, increasing the chance that the most evil option on the ballot will win.
So in summary, voting=lesser of 2 evils, not voting=increased odds of greater evil winning. In this light it could be asked whether failing to vote is morally right in this situation.