Principle #1 - We cannot directly support intrinsically evil actions. Abortion is one of these actions. Even if a legitimate authority, such as the government, tried to force a Christian to help with an abortion, one could not. Within this principle we must make a distinction, which will be very useful to us:
*Material vs. formal cooperation with evil. No matter how hard you might try, there are situations were good an evil are mixed up and sometimes we get caught being complicit in an evil act.
When we "cooperate" in an evil act our cooperation can be either "material" (meaning well-removed from it) or "formal" (close to the evil act). An example should help understand - If you went shopping at a store because they had the best prices in town on books and while you were checking out you noticed that they also sell pornography, you could still licitly still shop at the store, if you don't intend to support the selling of porn. But, you are still in material support of the evil. A good rule to follow is that while material cooperation may be licit, we want to be as far-removed from formal cooperation as possible. So, if there is another option of shopping somewhere with similar prices and selection, which doesn't sell porn, we might shop there instead. Remote cooperation is licit because we don't intend to cooperate in the evil act and if we did intend to coooperate with evil it is no longer a licit act.
There are times when we are not remote from the evil at all. A clinic employee who assists in an abortion is formally cooperating in evil, and the act is not remote at all.
The less remote the cooperation, the more we should seek to do something different. Once it is no longer remote from an evil, we ought never participate. There are more distinctions we can make within this principle, but we will allow this short answer to suffice.
Principle #2 - We must pay our taxes in support of the common good as an obligation to the legitimate authority of the state to take care of citizens. Because we must "render unto Caesar" we must pay taxes to the government. The Catechism says:
"Submission to authority and co-responsibility for the common good make it morally obligatory to pay taxes, to exercise the right to vote, and to defend one's country" - CCC, 2240Conclusion - Based on both of these principles, paying taxes to a legitimate government (which the US qualifies for) is an obligation we must follow. If the government uses taxes to help pay for abortion, we would not support such a policy (and actively seek to change it), we would be material cooperating in the evil and such an act is licit. Thus, we should still pay our taxes. We can never intend to support abortion with our tax money for it to remain a licit act.
If this doesn't satisfy, then think of it this way - Jesus demanded that his followers pay taxes to Caesar and the Roman government was supporting evil acts during that time (e.g., torture, unjust war, worship of pagan gods, etc.). The situation is almost identical to what we face in the question above.
I hope this helps. Peace to you and good luck filling out your income tax forms next year.