Story highlights Republican rules will likely limit the delegates' choice to either Cruz or Trump, David Gergen says

There are 16 primaries and caucuses remaining

David Gergen is a senior political analyst for CNN and has been a White House adviser to four presidents. A graduate of Harvard Law School, he is a professor of public service and co-director of the Center for Public Leadership at the Harvard Kennedy School. Follow him on Twitter: @david_gergen. The opinions expressed in this commentary are his.

(CNN) With his decisive victory in Wisconsin, Sen. Ted Cruz has not only shaken up the Republican presidential race, but heading into the homestretch, he has suddenly become the odds-on favorite to win the nomination in Ohio.

With 16 primaries and caucuses remaining, Donald Trump has to win 70% of the delegates to secure the 1,237 needed to win a first ballot at the Republican convention. Several states are coming up that are more favorable territory for Trump than Cruz, especially New York and Pennsylvania where Trump still has significant leads.

Even so, winning more than two thirds of the remaining delegates is a daunting challenge for him. In the 36 primaries and caucuses leading up to Wisconsin, Trump won only 46% of the delegates. And now he heads down a tough homestretch with Cruz seizing the momentum.

In a year crammed with surprises, no one can say for sure what will unfold in Cleveland, Ohio. But there are two likely outcomes: First, Cruz and Trump have each vowed to vote against a change in the GOP's Rule 40. That's an obscure provision that requires any candidate to win at least eight primaries and caucuses before he or she can be nominated.

Trump and Cruz will be the only two people in Cleveland with that distinction. They should also have enough delegate strength between them to block a rewrite of Rule 40. In other words, potential candidates like John Kasich, Paul Ryan and Mitt Romney won't be eligible even if many delegates think them likely to fare better against Hillary Clinton -- the race could narrow to Trump vs. Cruz.

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