Tens of thousands of gay rights supporters marched through Mexico City on Saturday with both hope and some nagging doubts about the prospect of a presidential election victory by leftist front-runner Andres Manuel Lopez Obrador next weekend. A Lopez Obrador win on July 1 would be a watershed in Mexican politics, but the electoral marriage of convenience he has forged with a socially conservative party worries some of his liberal backers about his commitment to gay rights.