One of the topics that seems to generate a fair amount of discussion every election cycle among Catholic blogs is what sort of moral act voting is, and specifically to what extent, as a voter, one takes on responsibility for whatever ways in which the candidate one votes for might misbehave once in office.
If I may simplify a bit, there seem to be two basic schools of thought on this issue:
One school holds that in casting a ballot one takes on responsibility for the actions of the person you vote for (if elected) and thus you should be very, very careful about supporting anyone who doesn't show absolutely upright moral principles. Given that the leader of any country will often end up committing a range of sins -- either through falling to the temptations of power or through feeling it necessary to perform immoral actions as the only apparent means available of protecting the nation entrusted to his care -- this usually ends up resulting in members of this school of thought taking a "pox on both their houses" approach to politics.
The other school holds that the act of voting in a representative democracy is simply one of selecting between the options before one. Members of this school do still often hold that voting for someone with certain stated policy goals is very, very hard to morally justify, given an alternative who doesn't (example: members of this school would typically argue it's very hard to justify voting for a pro-abortion candidate over an anti-abortion one) but they hold that it's important to vote for what one believes to be the better of the candidates, warts and all. Members of this school even sometimes argue that it's hard to justify voting for a candidate with very little chance of winning (in US politics: any third party candidate) since this means not helping the odds of whichever is the better of the major party candidates against the other.
Both schools have some points, as is invariably the case. I think the first school does make a valid case that once we have picked "our candidate" we tend to defend the actions of that candidate more than we should, perhaps turning a blind eye to things we could criticize in a member of the other party. However, I tend much more towards the approach of the second school. My rationale is that we are going to be ruled by someone no matter what. Given that, one can choose either to have some influence on whether it is a better or worse candidate who fills that role (choosing among the options available) or one can sit back and refuse, but end up getting ruled by them nonetheless. All things considered, I think it guards the common good more to encourage serious Catholics to have a voice in that process than to fear the participatory element of democracy and leave the selection of our leaders to those who do not share those qualms.