I notice some confusion on whether Cruz was wrong in backing out of his vow to support Trump. I'll make a few observations, not about Cruz's motivations (which I'm not privy to), but the general principle.
In biblical ethics, vows (oaths, pledges, promises) aren't absolute. Consider Num 30:3-5, where a father can annul a minor's pledge.
There's also the distinction between lawful and unlawful vows. In the nature of the case, there can be no moral obligation to keep an immoral vow. Unethical duties are self-contradictory.
As a rule, we shouldn't make promises we have no intention of keeping. But humans are shortsighted. We sometimes make a promise under one set of circumstances. We wouldn't make the same promise if the circumstances were different. So what happens when the situation changes between the time of the promise (vow, oath, pledge) and the time to make good on the promise?
Parents sometimes make promises to kids which they intend to keep at the time, but then unforeseen circumstances intervene to render the promise impractical. That's the nature of being a creature: you don't know or control the future.
There's no moral obligation to keep a rash vow. It was a mistake to make a rash vow in the first place. To keep it compounds the initial mistake. Jephthah is the classic example. It was foolish for GOP candidates to issue the nominee a blank check.