Choosing a candidate whose personal life is gravely sinful is itself a decision the effects of which need to be taken into consideration. It would not be lawful to choose an unworthy candidate when there are candidates whose lives are not mortally sinful running if their policy proposals were not drastically different, e.g. during the republican nomination. See below.
Someone who claims he could support Trump, but could not support a candidate like Cruz whose life was not sinful and whose policies were more or less equivalent if not more consistent, is simply mistaken. Cruz was more consistently anti-interventionist than Trump and also didn't suggest we could torture innocent families in the hope that they may provide some incriminating evidence about their relatives' link to terrorism. That's classic ends-justifies-the-means, is not licit, violates the Geneva convention, and more importantly, the teaching of the Gospel, the Angelic Doctor, and the Catholic Church. Trump is a a bad choice any way you look at it whose nomination should have been prevented. Almost any Republican with a serious record of fighting Planned Parenthood and abortion whose policies were broadly non-interventionist would be better. Cruz, or even Pence, is also more credible on appointing strong pro-life Supreme Court justices, judging based on their past record.
Fr. Cranny says,
"When unworthy candidates are running for office, ordinarily a citizen does not have the obligation for voting for them. Indeed he would not be permitted to vote for them if there were any reasonable way of electing a worthy man, either by organizing another party, by using the “write in” method, or by any other lawful means. On the other hand, it would be licit to vote for an unworthy man if the choice were only between or among unworthy candidates ... Since the act of voting is good, it is lawful to vote for an unworthy candidate provided there is a proportionate cause for the evil done and the good lost. This consideration looks simply to the act of voting itself and does not consider other factors such as scandal, encouragement of unworthy men, and a bad influence upon other voters. Obviously, if any or all of these other factors are present, the excusing cause for voting for an unworthy candidate would have to be proportionally grave ...
As a practical point it may be remarked that at times a citizen may have to vote for an unworthy man in order to vote for a worthy one, e.g., when people have to vote a straight party ticket, at least in a primary election when the “split ticket” is not permitted. However the good to be gained would have to outweigh the evil to be avoided, or at least be equal to it. In his Casus Genicot sets up a case of an election between a liberal and a Communist. To avoid scandal the citizen should give reasons for voting for the liberal. One does not support the evil candidate but simply applies the principle of double effect."
It is permissible to vote for Trump because Hilary is evil, because what she'll do as president is certain, very grave and a serious worsening of the common good - the importance of preventing that allows taking the step of a "holding one's nose" vote for Trump. The evil Trump stands for on gαy, lgbt rights etc should not be supported nor should even the appearance of doing so be given. It's good Trump has pledged to sign the pain capable act that prevents abortion after 20 weeks, and to nominate from a list of pro-life justices, but that's about it. Conservatives should continue to put pressure on Trump and see to it that he fulfils what he's pledged when he wins.