While Trump has had the nomination locked down for weeks, he has now reached the threshold of 1,237 delegates with the help of previously uncommitted delegates who now support his candidacy. A handful of states, including the large prizes of California and New Jersey, will hold the final primaries on June 7.
While Trump will not formally accept the party's nomination until the delegates cast their votes on the convention floor in July, crossing the threshold effectively puts to rest any remaining suspense about the possibility of a messy and contested convention. For months, drama and tumult have rocked the Republican Party, as a fervent anti-Trump movement launched a full-on onslaught to derail his candidacy.
But the flurry of "Never Trump" activities, including a slew of negative ads against Trump and efforts to draft a third-party candidate, ultimately proved unsuccessful.
Speaking at a news conference in Bismarck, North Dakota, Trump reveled in the fact that he had clinched his party's nomination before Hillary Clinton.
"Here I am watching Hillary fight and she can't close the deal," Trump said.
Thursday also marks an extraordinary accomplishment for Trump.
The launch of his unlikely presidential campaign
last summer was met with widespread ridicule. His wholly unorthodox campaign and rhetoric not only unsettled and alarmed GOP party leaders, but also convinced many that the first-time political candidate would soon lose steam. But if his campaign was initially underestimated by the party and political media, it soon became clear that Trump's anti-establishment, populist rhetoric had struck a nerve across the country.
"No one in American history has moved from a June 16 announcement to a May 26 winning of a majority," former House Speaker Newt Gingrich, rumored to be among the candidates on Trump's vice presidential short-list, wrote on Twitter Thursday. "Trump's achievement is remarkable."
But even as Trump and his campaign celebrate, they now have a more daunting task ahead: defeating Clinton in the general election.
Trump is up against historically high unfavorability ratings and faces immense challenges as he looks to broaden his appeal and base heading into November. The primaries revealed his weaknesses with constituencies like women and minorities.
And since essentially locking up the GOP nomination after the Indiana primary, Trump has confronted various setbacks. He must still to win over key GOP leaders, including House Speaker Paul Ryan,
and while he's begun to build out a national finance team, many major party donors and financiers still remain opposed to his bid.
Trump divulged part of his general election strategy at a rally Thursday in Billings, Montana, just hours after crossing the 1,237 delegate threshold.
"What I'm going to do is I want to focus on 15 or so states, because we have to win, and I want my energy to be put in the states where it could go either way," Trump told the crowd in solidly Republican Montana, which Trump made clear was not included in his list of swing states.
While those states likely include traditional battleground states like Florida, Ohio and Pennsylvania, Trump also said he will spend significant chunks of time in states that have trended solidly blue in recent presidential elections.
Trump said he believes California and his home state of New York could be in play, for example. And he argued that a "traditional Republican" nominee would not put those states in play, as he believes he has.
The Trump campaign did not respond to a request for comment asking for the list of 15 states it plans to target in the general election.
Trump comes back to Washington this weekend to speak at the Rolling Thunder Memorial Day event.
Meanwhile, Trump's campaign has been fraught with internal divisions — culminating in the sudden departure of top campaign aide Rick Wiley this week — that have highlighted the grave challenges Trump faces as he attempts to transition into a more professional general election candidate.