In order for Trump to secure the vaunted 1,237 delegates needed to win the nomination, he has to secure 60 percent of the remaining 1,134 delegates.
That could mean a contested convention whether he likes it or not.
Donald Trump leads in the race for committed delegates with 621, followed by Texas Sen. Ted Cruz at 396, and Ohio Gov. John Kasich 138.
Before ending his candidacy, Florida Sen. Marco Rubio had 168 delegates, who remained committed to voting for him at the convention until he formally released them from their commitment.
Another 21 delegates are either committed to other candidates or uncommitted.
But that delegate count is far from the vaunted 1,237.
"No candidate will win 1,237 delegates," declared a memo from Kasich's chief strategist John Weaver Tuesday night, predicting that more states favorable to Kasich would be selecting delegates between now and June. Weaver also predicted his candidate would end up going to Cleveland with "1,000 delegates."
If no candidate wins a majority on the first ballot in Cleveland, the convention will ballot until someone does secure that majority.
Under party rules, while delegates are bound on the first convention to support the candidate they are pledged to in a primary or caucus, they are free agents on subsequent ballots and can vote for whomever they wish. That could even mean someone who had not been seeking the nomination, such as House Speaker Paul Ryan.
"It's very possible but we have to go farther down the road to see if that's going to happen," author and political scientist Stephen Hess of the Brookings Institution told me. Hess, has the distinction of being one of the few living Americans of having attended the last two contested Republican conventions in 1952 and '76.
He added that a 2016 convention with its nominee unclear "would be a contested convention, not a 'brokered convention' as the press keeps calling it. 'Brokered' means it would be determined by party leaders. Contested is what it says it is. And all of you reporters and all the political junkies would love it!"
Ted Cruz would have to win 80 percent of the remaining delegates, while John Kasich would have to secure the mathematically unattainable figure of 110 percent.
In 1976, supporters of Ronald Reagan for President arrived at the Kansas City convention with their man slightly trailing incumbent President Gerald Ford in delegates. One ploy they advanced was to place in nomination the name of New York's Sen. James Buckley (who was neutral in the presidential race) in the hope he could draw enough votes to deny anyone the nomination and thus force the process onto a second ballot.
After briefly considering the nomination, Buckley — who found fellow New York Republicans furious at him as he was facing a difficult re-election race — announced he was running for the Senate and nothing else.
The last time a Republican convention went beyond one ballot was in 1948, when New York. Gov. and long-presumed front-runner Thomas E. Dewey won on the third ballot.
The last time a Democratic convention went beyond one ballot was 1952, when Illinois Gov. Adlai Stevenson was genuinely drafted on the third ballot after insisting he was not a candidate.