The Church has a responsibility to educate voters regarding the principles behind just voting, since in a democracy, the people have a responsibility to vote properly.
The fundamental purpose of the State is the advancement of the temporal welfare of its citizens. It necessarily follows that the primary duty of the State is to defend the lives of its citizens. Any politician who advocates laws tolerating the killing of innocent people is unworthy of holding any public office.
Further details are available here: Voters Guide for Serious Catholics
The poster's argument is a typical appeal to Catholics to ignore the conscience that was shaped by Catholic teaching.