UPDATE: Sounds like he's going to sign it (via Politico):
A close associate tells POLITICO that Donald Trump plans to sign a loyalty pledge Thursday that would bind him to endorse the Republican nominee, and would preclude a third-party run.
Seeking to avert a 2016 disaster, the Republican National Committee on Wednesday challenged every GOP presidential candidate to sign a pledge not to undertake a third-party bid under any circumstances.
The challenge, confirmed by multiple campaigns, is aimed squarely at Donald Trump. And the timing of the pledge suggests an agreement has been reached.
While he is leading the packed Republican field in early polls, the billionaire businessman last month repeatedly threatened to launch a third-party bid — leaving open the possibility even at the GOP's first presidential debate last month — should he fail to claim the Republican presidential nomination. Such a decision would make it all but impossible for the Republican Party to win the White House in 2016.
RNC officials have been working privately with Trump's campaign for several weeks to avert such a scenario.
Over at Politico, they noted that the pledge goes something like this:
“I [name] affirm that if I do not win the 2016 Republican nomination for president of the United States I will endorse the 2016 Republican presidential nominee regardless of who it is,” the pledge reads. “I further pledge that I will not seek to run as an independent or write-in candidate nor will I seek or accept the nomination for president of any other party.”
They added that two campaigns reportedly spoke with RNC Chief of Staff Katie Walsh over the proposed pledge. The publication added that RNC spokesperson Allison Moore refused to comment, while the Trump campaign didn’t return their calls.
The Donald has switched positions on this issue. In July, he told The Blaze’s Dana Loesch that his “total focus is to run as a Republican and to win.” The next day, he said that he might run as a third party candidate if the RNC wasn’t “fair” to him. Yet, he seemed to have a change of heart again earlier this month when ABC News reported that the billionaire was reconsidering taking a no third party run pledge.
So far, the RNC has been somewhat fair to Mr. Trump. Despite his criticism of the first GOP debate, his support has grown; he’s way ahead in the polls; and he’s thus far the frontrunner for the nomination. At the same time, his rise is something of an anomaly.
He’s done everything that you’re not supposed to do when running for office, but he’s increased his numbers. It’s puzzled the media establishment. It’s left the consultant class speechless. And the Republican Party seems to be thinking that it's time to at least make moves towards a no third party run pledge since Trumpmentum doesn’t seem to be on the verge of ending anytime soon.
If Trump feels like he’s going to win, why not take the pledge? That’s what you do when you’re a member of a party, especially when your opponent is at the helm of one of the most vicious political machines in American politics. He can say that he’s a proud Republican, and he would garner the support (at least in spirit) from the rest of the field (who will also probably take the pledge) if he clinches the nomination.
Yet, we still have a long ways to go until Iowa. A lot can happen, with most in the establishment hoping that Trump is saddled with a pledge he hypothetically signed–and a crumbling campaign as voters in the early voting states begin casting their ballots. Then again, this is Donald Trump. He could break the pledge and still finance his own campaign with his own assets, though that would certainly guarantee Hillary Clinton moving into the White House by January of 2017.
Even with Trump signing the pledge, Politico quoted his campaign manager, Corey Lewandowski, saying that we can't "expect" anything from the Donald. So, as with anything in Trump world, it's all up in the air
Last Note: More on the risks and rewards for both sides with this pledge from Ed over at Hot Air:
This suggests that Trump’s taking this campaign seriously, perhaps more seriously than many thought and still think at the moment. Still, this cuts completely across his main draw. Voters flock to Trump because they’re angry at the Republican Party, which has won control of Congress and so far has seemingly done little with it. Locking himself into the GOP, and especially making agreements with its senior leadership, might well undermine that rogue quality which has thus far kept voters from caring too much about his history of left-of-center policy stands. It’s a risk for Trump, one that might just put him in the one category fatal to his populist aspirations: politician.
It’s a risk for Priebus, too, who will have to offer the RNC’s crown jewels to what might end up being a rogue prince. Given the mood among the base, Priebus can hardly risk shunning Trump, and will have to make do with a public pledge that has all the value and substance of “Trust me” in a business deal. Will a broken pledge matter to those flocking to Trump? Not now, certainly. It might when the primaries roll around and voters have to stop dating and finally marry a candidate, but that’s not a slam dunk either. Pinning a pledge on Trump will prove to be a very difficult act.