A contested convention, which allows delegates to propose alternatives and requires rounds of voting until one wins, occurs when no candidate gets the number of delegate votes necessary to become their party's nominee. In the Republican Party's case, that's 1237, and Trump currently has 1013, according to The New York Times. There are currently 445 unpledged delegates remaining. So, even if only about half the remaining delegates vote for Trump, the nomination is his.
The fact that he's the only remaining candidate, though, doesn't mean all delegates must vote for him. They're still free to vote for Cruz, Kasich, or even Rubio, Mark Weston, author of The Runner-Up Presidency: The Elections that Defied America's Popular Will, told Complex. But he doesn't think they will.
"The ones who are committed to Cruz and Kasich and Rubio can stick with their person, and the ones who are uncommitted can vote for whomever they want," he said, but "there are so many delegates who haven't committed yet. They're going to go for the winner."
Kasich's exit sealed the deal for Trump, he added, explaining that the Ohio governor could have gained momentum and taken California's votes away from Trump if he'd won in Oregon.
Hans Noel, Associate Professor of Government at Georgetown University, told Complex certain states have rules stating that the Republican delegates must vote for whomever their constituency votes for, but they're not always honored. When there was talk of a contested convention, some states were telling delegates that they could vote for whomever they wanted. Now, he said, "the question is, 'how much are they going to respect or require those rules?'"
"Weird things can happen," Noel said, "but at this point, it doesn't look like the party is trying to use some of those backdoor strategies."
So, there you have it: we're headed toward a Clinton-Trump face-off. Or, as Weston put it, "The Casino Owner has won the Republican nomination."