Proportionally was well explained by arch bishop ch*putz ( I can't spell his name).
If , when you get to heaven, and meet the soul of an aborted child, you think that they would agree with your vote, then you are weighing the abortion issue proportionately. Weather or not you are correct about that is a different issue.
Noel, a mortal sin requires full knowledge. So there are many Catholics who will commit a grave sin by voting for oboma, but because they are not well informed the sin may or may not be mortal , depending on many factors including if they intentionally avoided opportunities to find out the truth.
A sin is grave regardless of the intent of the sinner. That is why joe biden has be asked by his bishop to refrain from taking communion , because the bishops believes he is obstinate in committing grave sin. It is not a judgment as to weather or not he is going to hell, because only God knows if it is mortal sin.