Which one would he be likely to endorse this time around?
I do not know if Benedict XVI would endorse McCain, but he most certainly would not endorse the candidate who killed the "Born Alive" legislation in the Illinois state legistlature, the one who clings to abortion-on-demand without remorse.
This is key. Not only is Obama simply pro-abortion (evil as that is), but he is pro-death. If he supports killing a child outside of the womb, what is to say that it is not proper to kill the elderly or disabled by euthanasia.
Catholics should not support Obama.
Blacks should not support Obama, as it is their race that is hurt the worst by abortion (yet you will never hear Sharpton or Jackson say that).
As to whether it is sinful, I'm not completely sure. I would suggest that in many cases (Catholics who have received any sort of moral education whatsoever), the answer is probably yes.