COACHELLA, Calif. — Both presidential campaigns agree that seven swing states are likely to determine the outcome of this year’s election. California, which has not voted for a Republican in a presidential race since 1988, is not one of them. But that did not prevent former President Donald Trump from heading there anyway Saturday evening to hold a rally in Coachella, which is better known for its annual music festival with headliners like Lana Del Rey and Bad Bunny than it is for being a stop on a presidential campaign trail. It was an unusual choice 24 days before the election.