With more than half the states having now held their nominating contests, Donald J. Trump and Senator Ted Cruz are quietly directing their attention to a second, shadow election campaign — one that is out of sight, little understood, but absolutely critical if Republicans arrive at their national convention with Mr. Trump short of a majority of delegates.

This parallel campaign is to select the individual delegates who will go to Cleveland in July for what could be the first contested convention in American politics in more than 60 years. Chosen through a byzantine process in each state, most of the delegates will become free agents if no one wins a majority on the first ballot.

The mere prospect that delegates could deny Mr. Trump the nomination led him to predict Wednesday that violence could erupt in such a situation.

“I think you’d have riots,” Mr. Trump warned. It seemed no idle speculation after the mayhem at Trump rallies. Speaking to CNN, Mr. Trump said he still expected to reach the majority of 1,237 delegates needed for a first-ballot nomination before the end of the nominating season. “I’m a closer,” he said. “I get things closed.”