Voting for a pro-abortion candidate a mortal sin?

[USTNoah]

I have a question for an apologist. According to Fr. Stephen Toroacco, in his "A Brief Catechism for Catholic Voters" (Copyright 2003 by Leaflet Missal Company): "To vote for such a candidate simply with the knowledge that the candidate is pro-abortion is to become an accomplice in the moral evil of abortion.

If the voter knowingly and freely assents to his or her status as such an accomplice, then the voter sins mortally." My question is, is it a mortal sin to vote for such a candidate? My main question is, is on an accomplice in a moral evil merely by voting for a certain person? If so, what documents (i.e. CCC, encyclicals, apostolic letters) support this premise?

I most certainly agree that abortion is a moral evil, and to be an accomplice in such is a mortal evil - but to vote for someone does not seem to be on the same moral level.

Please explain!

Thanks,
USTNoah