Donald Trump continued this week to lose ground to Sen. Ted Cruz in the race for second-ballot delegates, whose votes will matter in a contested convention. In the latest setback, a caucus in one of the seven congressional districts in South Carolina picked delegates who are likely to support Cruz if the national convention goes to multiple ballots. The process illustrates the complex system by which delegates are actually chosen in many states, which Trump, a political novice without a strong national organization, has been denouncing as rigged against him.