In Jamelle Bouie’s Slate.com post “Why the RNC’s Loyalty Pledge Was a Huge Mistake,” he explains why Donald Trump’s signing the pledge sets the stage for an even worse disaster:

…A Trump nomination is so unlikely that it’s not the actual nightmare for the Republican Party. The nightmare is a third-party run, where Trump gets himself on the ballot in all 50 states, and siphons white voters from a GOP that needs white turnout to win national elections.
…On the surface, this is an important victory for Republican leaders. But look carefully, and it’s somewhere between a disaster and a catastrophe. Trump hasn’t just bound himself to the RNC, the RNC has bound itself to Trump and put pressure on other candidates to do the same. Let’s say Rubio wins the primary and becomes the Republican nominee. Thanks to the pledge, he’s linked to Trump, and Democrats can run wild with guilt-by-association. By the end of the campaign, Trump might be the face of the Rubio campaign, as much as the Florida senator himself.

I’m not sure any GOP nominees would be all that bound to Trump just because of the pledge. It’s more about Trump sliming the whole party and his misogynistic, Latino-bashing and general boorishness becoming the new face of the Republican Party — regardless of the pledge. He sucks up all of the media coverage to the point where the word “Republican” conjures up a caricature of Trump’s head. He’s a wet dream, not only for cartoonists and late-night comedians, but also lazy TV reporters and commentators. The other GOP presidential candidates can’t get arrested as long as he is around.
But I think Bouie is dead right about Trump’s fidelity to the pledge, or anything he says, for that matter. This is a man who thinks it is perfectly alright to contradict himself 180 degrees within a couple of days, if not hours. As Bouie explains:

That’s the disaster. The catastrophe is that there’s nothing to hold Trump to the pledge. As soon as it becomes inconvenient, he can break it. And because he’s untethered from the institutions of the Republican Party, Trump has nothing to lose from breaking the pledge. Indeed, anything he gains from signing–the imprimatur of the GOP and commitments from other candidates–is almost irrelevant to his appeal as the “outsider” who understands the world of the “insiders.” The only thing that ties Trump to his word, on this score, is the promise of official “respect.” For a man of Trump’s ego, that’s weak binding.

Bouie shares a reminder of Ross Perot’s waffling about his independent candidacy intentions in 1992, and Perot was a guy with some actual principles. Remember Perot prattling on about being drafted by “the volunteers”? Something similar is all the cover Trump would need to trash his pledge, argues Bouie quite convincingly.
“The people are calling, and I must answer the call,” has been leveraged by many a demagogue down through the ages, and it’s not hard to imagine Trump playing that card with gusto. Some will grumble if he breaks the pledge, but few of his potential supporters would hold him accountable. They got on the Trump bandwagon less because they admire his consistency and integrity, and more because they wanted to root for a rogue rhino smashing up the crystal.
What “the pledge” does for Trump is buy him a little breathing space. His GOP opponents can and will still attack him. But he has disarmed the “not a real Republican” argument to a degree, at least for the time being. It secures the possibility that he can win the GOP nomination, but does nothing much to insure that he won’t reneg on the pledge and run as an independent. As Bouie concludes,

If anything, the loyalty pledge enhances his platform. He can run his campaign–touting Social Security and condemning illegal immigration–and when he loses the nomination, he’ll have the audience and support he needs to make an independent run. Whether Priebus knows it or not, he’s been played, and it’s going to hurt.

Priebus had to do something, and the pledge also gives him a little cover. But no one should bet that it will be honored. Trump may fool us all and gracefully bow out when the time comes, but at this point that’s not a bet for the smart money.
Dems have to run their best campaign, regardless of what Trump does. He may end up a king-maker, or worse, gulp, a king. Let the media continue to obsess about Trump’s distraction du jour, all the way to November of next year. For Dems, however, the challenge is to get focused on mobilizing their base and honing the message that there is only one party that represents adult America, and it is not the party Trump currently leads.