Through much of the primary process, Donald Trump promised, like every other major-party presidential candidate in the past four decades or so, to release his tax returns. Then he said he was being “audited” for some years so he could not release returns. Now he says he will not do so before the November election, setting up a bizarre situation in which he and voters will likely know about his running mate’s tax history but not Trump’s.

This is not only unprecedented, but it also goes to the heart of the Trump campaign. He’s sold himself as a successful billionaire, one who wants the United States to “win” and who deplores foreign countries and illegal immigrants “stealing” our jobs. But what if his tax returns show he is much poorer than he has ever admitted, has shady foreign investments and tax havens, has given little in charity (or only to left-wing groups), or in fact has been audited and found to have violated tax laws? The entire Trump image crumbles.

Moreover, there is a fundamental principle of fairness at work here. He said he would do release his returns, and people voted for him with the confidence that we would dispel any notion of a hidden bombshell. Delegates were selected and pledged to him on that basis. Now, the GOP faces the potential for its nominee, through leaks and hacks, to have damaging tax records released — when it is too late for the party to do anything about it. This is unconscionable. In fact, it’s an issue the #NeverTrump forces have been raising:

Delegates can do something about this. Indeed, they are obligated to for the sake of the party.

Remember, as one delegate expert put it, ““The only way delegates will be bound is if, when they vote on the rules of the convention, they vote to bind themselves. Otherwise, they’re not.” Put differently, delegates control their destiny. Curly Haugland, the delegate guru, has a million maneuvers and potential rule changes up his sleeve for challenging Trump. Now he has a simple, effective one: Pass a rule that says the nominee, who promised to release his taxes, must do so in order to have his name put in for nomination. Such a rule could easily pass when you consider there are nearly 900 delegates pledged to other candidates and a great number of Trump delegates are reluctantly so. It is entirely possible to come up with a majority of delegates to insist on adoption of the rule. In fact, the Republican National Committee, which has refused to look out for the party’s long-term reputation, should insist on it as an institutional safeguard.

As John Fund wrote months ago:

Republican voters, GOP officials, and all Americans should demand that Donald Trump release his tax returns, something he refuses to do with the flimsiest of excuses. If he doesn’t release them, no one should be surprised if a leak of the juiciest details comes from the Obama administration before the November election. And the odds that anyone in the government would pay a penalty for that? Ask Lois Lerner, the comfortably retired former IRS official at the heart of the scandal involving discrimination against conservative non-profit groups.

The principle is simple: If Trump wants delegates to keep their promise (pledge) to support him, he must keep his promise to release tax returns to them. Moreover, those who have said they would support him, including members of Congress, former contenders, and other prominent elected officials, should make clear: Their support is conditioned on his releasing his tax returns. Moreover, delegates have any easy option if they cannot get a rule change. Fund explains, “If Donald Trump won’t release his tax returns prior to the GOP convention, the delegates pledged to him on the first ballot should abstain from giving him their votes. Other than their vote not counting, there are no realistic consequences for any delegate doing so on the first ballot. A few states make breaking the first-ballot pledge rule a misdemeanor, but no one is ever prosecuted. In theory, state leaders could exact political retribution but such discipline is rarely exercised.”

Trump’s cavalier attitude toward policy positions, other Republicans and ultimately the voters is already fueling a full-fledged third-candidate movement. A spokeswoman for the group Conservatives Against Trump tells me, “We are quite optimistic that we will have a candidate soon who commands instant respect, and who will be a strong alternative to the other two major candidates.” She continues, “We have considered current and former Senators and Governors, retired Generals, and highly successful businessmen. We are also preparing an operation to get such a candidate on the ballot across the country.” The latest Trump news should only spur that effort.

Republican delegates, officials and activists face three choices: let Trump go back on his word, setting the party up for disaster; pass a rule to hold him to his promise; or get behind a third-candidate push as a backup if, as is now possible, Trump implodes over this or some other issue.