For the sake or argument, let's agree that the Church's teachings on abortion and marriage are De Fide. That still does not dictate how one conscientiously participates in a democracy. I can accept that candidate X is wrong about abortion but, on balance, still the better choice. 

Forty-plus years of voting for so-called pro-life candidates has gotten us exactly bupkis. On the other hand, under Obama administration policies (for example), abortions actually decreased. The point is, one can be opposed to abortion, as I am, and also believe there is no legislative or electoral solution to the problem.

It may ease the consciences of some to think that they do their moral duty by simply voting against all pro-choice candidates, but that policy has put into office some truly repulsive and self-serving characters. The current occupant of the Oval Office comes to mind.