Would this prevent a Catholic lawyer from defending Dr. Gosnell? If not, is there a difference between a lawyer representing a client and a politician representing his constituents?
A lawyer has a responsibility to secure the best interests of his or her client. Furthermore, the scope is limited to a particular case or set of cases.
A politician who votes to legalize, expand, promote, or especially fund abortion, has gone beyond the actions analogous to "representing a client." Such acts are cooperation in the act of abortion itself: the active promotion of an evil. The politician's acts make his an accessory or accomplice.
A more accurate analogy would be, if Gosnell's lawyer said "Yes he did kill all those babies after 24 weeks, and yes he did kill newborns by, effectively, decapitating them: and I say that's all right, I say that's justified, I'm going to try to make this more widespread, and I paid him to do it."