The assumption that the most effective way of working to avoid abortions is to have laws making abortion illegal is that, an assumption.

It is quite possible to argue that it is liable to be ineffective and damaging, and that a more promising approach is to work to get rid of the factors in society which make abortion seem the best choice, or even the only choice, for many women.

Thus a candidate who is opposed to the introduction of laws restricting the availability of abortion may support other policies which would offer the hope of reducing abortions overall.   And a candidate who is in favour of laws against abortions may support policies which would overall rend to increase abortions.

In those circumstances, anyone voting on the basis of a moral opposition to abortion would choose to vote for the former candidate. And it seems likely that millions of Catholics in the USA took that view, and voted for Obama in perfectly good conscience. No priest or bishop has the right to override that conscience - and that is totally orthodox Catholic teaching.