Donald Trump pledged on Thursday not to run for president as an independent candidate if he loses the Republican nomination, an announcement sure to disappoint Democrats who've hoped he'll stick around through November 2016 no matter what.

"The best way for the Republicans to win is if I win the nomination and go directly against who they happen to put up," the mogul said at a Trump Tower press conference. For that reason, he said, he'd signed an RNC pledge stating that if he doesn't win the party's nomination, he'll endorse whoever does win — and forgo any independent or write-in bid.

Trump wants this announcement to suggest he's increasingly confident about his lead in the polls, and increasingly serious about trying to win the nomination.

Yet, though he's spinning it as a win for himself, he didn't really have much choice in the matter. Because in the important state of South Carolina, Trump won't even be let on the primary ballot unless he pledges to back the GOP nominee.

Trump is finally taking an independent run off the table

Previously, Trump has dangled the possibility that if he felt he was treated "unfairly" by the party establishment, he could mount an independent candidacy in the general election. The prospect was terrifying to Republican elites, since most polls showed that a three-way race including a Republican nominee and Trump would result in an easy Democratic win.

And last month, the very first question at Fox News's Republican debate was designed to put Trump on the spot about whether he'd pledge to forgo an independent candidacy — and he memorably refused to do so, saying he'd have more "leverage" over the party establishment if he didn't.

Now, though, after consultations with the RNC and its chair, Reince Priebus, he's finally opting to bite the bullet. Priebus, Trump said, "has been extremely fair, [and] the RNC has been absolutely terrific over the last two month period. And as you know, that's what I've wanted, I wanted fairness." He said he didn't get anything in particular from the RNC in exchange for signing, except for assurances that he'd continue to be treated fairly. And all of the other GOP candidates are expected to sign the pledge too — meaning they're committing themselves to support Trump if he wins the nomination.

But since South Carolina — a hugely important early primary state where Trump is leading the polls — requires all candidates to sign a pledge like this to make the ballot, Trump really didn't have much choice here if he wanted to seriously compete. And Virginia and North Carolina were in discussions about adding a similar requirement for their own primaries.

Of course, it's possible that Trump could simply break the pledge if he loses. He'd take some heat for it, but he could surely come up with some rationale for why he was treated unfairly, or how he just couldn't ignore his supporters' calls for him to run as an independent. But, he said at the press conference, "I see no circumstances under which I would tear up that pledge."

VIDEO: Donald Trump was coy when asked if he would run as an Independent