IMO, the President is the moral leader of America. When a politico supports abortion on demand he's a liberal. Giuliani not only supports abortion on demand, he opposes any ban on partial birth abortion. Giuliani also wants more restrictive gun control measures and an assault weapons ban. Promoting special rights for homos and creating a sanctuary for illegals is also liberal positions that conservatives take into consideration when picking any candidate for political office.
Is that in Article II, or has there been an amendment that I don't know about?
Stupidity. Liberalism, like conservatism is built upon a plethora of positions and leanings. A person, candidate or not, who agrees with one of those positions, even abortion, while disagreeing with a multitude of others will never be welcomed into the liberal fold.
Likewise, a pro-life candidate who was likewise anti-2nd Amendment, against lower taxes and anti-military would not be a conservative no matter how rabidly pro-life.