Party loyalty vs. marital fidelity
We hear every election that a candidate who loses a primary should endorse the winner, especially if that candidate has pledged to endorse the party's nominee. This exists at every level, but is especially intense in the race for President. I have said in the past and I continue to maintain that is not and should not be an ironclad promise that cannot be broken. Christians, especially, should never treat it like this.
The easiest comparison is to marriage. We know from Malachi 2:16 that God hates divorce, and when a man and a woman take a marriage vows they are not to dissolve them. But the Lord Jesus Christ said that it is possible to divorce a spouse in the case of adultery.
A political party is a man-made, secular institution. Marriage is a covenant created by God Himself, and that covenant existed at creation in the Garden of Eden. If one believes, as virtually all Protestants believe, that divorce is allowable under exceptions specifically granted by Scripture, then it is utterly absurd to claim that a promise to a man-made, secular institution cannot be voided.
Now, should we keep our promises, as Christians? Of course we should keep our promises. We see this all through Scripture. Jesus says that we should not swear to keep a promise, but that our yes should be yes and our no should be no. God never breaks His promises, and we should strive to emulate Him. We should also be honest. That means our promises to a secular organization do carry spiritual weight.
While Christians should take these promises seriously, there are occasions when that promise can be broken. The nominee himself can nullify promises of loyalty to him through his own behavior. But if you are treating a political promise to a secular organization as having more weight than a covenant established by God, you are putting man-made traditions above God and above Scripture. This is idolatry, and it is a very serious sin. Political parties are not God. We should never exhibit cult-like loyalty to a political party.