He'll do that at the convention, but first things first...
If Cruz goes into the convention with fewer votes and delegates than Trump, but comes out as the nominee, most Americans are going to consider him an illegitimate usurper of the people’s will.
They’re not going to give a damn about the party rules. They’re going to care more about the fact that Cruz couldn’t gather sufficient support from the people to win fair and square, but was given the crown by his party.
If you think the voters are going to be hunky dory with such a fundamentally unfair outcome, you’re dead wrong. It’ll be the end of the Republicans as a major party, if that’s the outcome of the convention.