Win an argument with a liberal? Well, as a teacher of Rhetoric I am well aware of the importants of facts, the art of persuasion, the beauty of masterful debate . . . but Democrats rely solely upon the ad hominem arguement delivered with highly emotional invective aimed at the personal destruction of their opponents. Trying to win any reasoned argument with them is folly. Reason is not in their lexicaon.
If Bush had better grades, then the grades won't matter.
If Kerry had better grades, it is proof is innate superiority.
You just can't win an argument with a liberal with facts. You have to force them into defending an indefensible moral position.
The process is simple. You ask a few base questions, such as premature births. Babies born after 6 months have a pretty good chance of living normal lives, and every week after 6 months they spend in the womb the better their chances. Don't let on that you're leading into abortion. Now change up the line of questioning to that of moral relativism. Are there basic morals that define a minimum good to society? Violence against others, when is it allowable? Theft of goods, when is it allowable? Destruction of property, when is it allowable? The idea is for them to take a moral stand on all those issues, and if they equivocate on these, then they just sound stupid. Now hit them with late term abortion. If they extend their moral relativism to late term abortion, then their lack of logic comes to the fore, and listeners will have to consider that there really are moral absolutes out there. I wouldn't say there are a lot of them, but there are enough of them such that most people will recognize evil when they see it.
Make sure they describe to you the procedure of late term abortion, and do not let them draw you into the trap of disagreeing with abortion as a whole until they've made answered your questions.
If you are 100% against abortions, be careful about being baited into becoming irrational. The question isn't one of abortions, it is the inclusion of late term abortions as a woman's right to choose. As long as Roe vs Wade stands, the best that conservatives can achieve is a limitation and restriction on abortions that are morally indefensible to moderates. The reason is that in order to make policy changes at the highest levels, you have to court the moderates, and a properly educated moderate should be against late term abortions.
If we can win that battle, we can put the necessary restrictions on abortions between 3-6 months, such that the fetus is accorded protections, only preempted by the mother's right to her own life. But simultaneously, we must push for American support in cultural changes including sexual responsibility, be it abstinence or contraceptives, so that unwanted pregnancies do not occur, and the question of abortions is a moot issue.