Updated at 7:15 p.m. ET
Donald Trump is "totally pledging" his allegiance to the GOP and promising not to mount a potentially damaging third-party bid for president.
The billionaire businessman said Thursday he had signed the Republican National Committee's "Loyalty Pledge," which says he will support the eventual nominee and not run as an independent or on another party line.
"I will be totally pledging my allegiance to the Republican Party and the conservative principles for which it stands," said Trump.
Trump made his announcement at the Trump Towers lobby in New York City after meeting with RNC Chairman Reince Priebus. While many had expected Trump would sign the pledge, one GOP source stressed this morning that, "With Trump, nothing is certain until it's final."
Trump was the only GOP presidential candidate to raise his hand at last month's debate to say he wouldn't pledge not to run as a third-party candidate.
The RNC said Thursday night that all 17 GOP candidates have now signed the loyalty pledge.
Trump said he has "no intention" of changing his mind about the pledge, because the Republican Party had treated him very fairly since his June 16 announcement.
"I see no circumstances under which I would tear up that pledge," he said.
But, Trump made clear, he intends to be the GOP nominee, repeatedly pointing to polls where he is leading by wide margins both nationally and in early states.
"The best way for the Republicans to win is if I win the nomination and go directly against whoever [Democrats] put up," said Trump. "And for that reason, I have signed the pledge."
Still wiggle room for Trump
Given Trump's history, though — even with his insistence he won't renege — it's not a given that he won't mount a third-party run — or that he'll even stand by this one. The RNC pledge isn't legally binding, and there's nothing holding him to it if he decides at a later date the party isn't treating him fairly.
The move appeared to be aimed at making sure he qualifies for early primary ballots. South Carolina, which holds the first primary in the South, has a similar pledge affixed to its ballot qualifications, which must be submitted by the end of September. Speaking in the Palmetto State last week, Trump demurred on whether he would sign it, insisting there was plenty of time.
But Trump seemed to be leaning toward signing it at an event last weekend in Nashville, Tenn.
"We're going to make a decision very soon," he told reporters. "And I think a lot of people are going to be very happy."
With the clock ticking and pressure mounting, Trump appears to be bending to the party — at least for now.
Bottom line — Trump has to sign these pledges if he wants to be considered a serious candidate in the short run. He wouldn't even be able to get on the GOP ballot in some important states like South Carolina and possibly Virginia and North Carolina, which are also considering loyalty pledges.
Whether those are enforceable is another matter.
There's a history of loyalty pledges in parties
Parties can determine who gets on their ballot. And when candidates have refused to sign a loyalty oath, they haven't been permitted to run in the primary.
In 2008, then-Ohio Rep. Dennis Kucinich sued to try to get on the Texas Democratic ballot after he wouldn't sign the party's pledge, but courts upheld the decision.
Richard Winger, editor of Ballot Access News, said while the primary ballot is enforceable, the general election is a different story.
Trump is stating, in good faith, that he won't run as any other party's candidate or an independent if he doesn't win the nomination, but there's still no legal muster behind such a pledge.
"They only have moral force. They can't enforce it," Winger said. "But it's embarrassing for people to give their word that they would not do something and then do it."
Some states do have sore-loser laws. If a candidate loses a primary, that candidate can't run as an independent in the general election. While that's mostly been in congressional races, some may also try to get it to apply to presidential elections, if it came to that.
But Winger said that could be a difficult argument, too. In his most recent issue of Ballot Access News, he notes that 34 states that do have sore-loser laws have precedents where they have allowed candidates to run again even if they lost the primary.
In some cases, that may apply only to an independent run. In 2012, former New Mexico Gov. Gary Johnson sought and lost the GOP nomination for president. Instead, he ran as the Libertarian Party's nominee and wasn't allowed on the Michigan ballot.
However, that was because he was running under another party's banner — had he simply been running as an independent, he would have been allowed.
And in states that hold caucuses instead of primaries, enforcement of a pledge could also prove difficult.
So while there is plenty of legal wiggle room for Trump, this is the surest sign yet that he is trying to play within GOP rules. His actions could engender goodwill from some in the establishment, even though many remain skeptical of his intentions.
With where he stands in the polls and with the size of the crowds he's drawing, party leaders have to recognize his staying power and that there is feasibly a chance he could be the Republican nominee.
For Trump, there is a bright side in signing the pledge, too. If he does win the GOP nomination, now his other rivals, who have badmouthed and beat up on him, will be the ones eating crow — and backing his candidacy.