Liberals are utilitarians in ethics, while conservatives are traditionalists. By their lights, liberals are behaving ethically if the goals they are advancing are (in their eyes) beneficial. If they played fair and lost, that to them would be unethical behavior.
One giveaway is that, depending on the occasion and the particular goal, liberals will use contradictory arguments (because, to them, the truth of the argument is irrelevant: it's merely a means to an end). For example, on abortion they borrow the libertarian argument that "the government must not interfere between a woman and her doctor" -- and then turn around and ban silicone breast implants!