TIJUANA, Mexico — The uncomfortable and dangerous rides atop freight trains are now in the past. So are the cold nights sleeping in parks, the hot days walking in the unforgiving sun and the unpredictability of the next meal or bath.

Yet for hundreds of migrants who arrived in this border city this week after a month traveling en masse across Mexico, perhaps the hardest part is to come. The hope of sanctuary in the United States had sustained them throughout the trip, and, for many, one person now stood in the way: the president of the United States.

“He doesn’t want anyone to enter,” said José Ignacio Villatoro, 20, who said he fled gang violence in Guatemala with his parents and three siblings. Mr. Villatoro was standing within sight of the border fence this week, weighing what he had been through and the effort that is still required.

“I’m thinking about how to enter because it’s not at all easy,” he said, looking at his shoes. “I really don’t know what’s going to happen.”