After six years of steep declines across the Southwest, illegal crossings have soared in South Texas while remaining low elsewhere. The Border Patrol made more than 90,700 apprehensions in the Rio Grande Valley in the past six months, a 69 percent increase over last year.

The migrants are no longer primarily Mexican laborers. Instead they are Central Americans, including many families with small children and youngsters without their parents, who risk a danger-filled journey across Mexico. Driven out by deepening poverty but also by rampant gang violence, increasing numbers of migrants caught here seek asylum, setting off lengthy legal procedures to determine whether they qualify.

The new migrant flow, largely from El Salvador, Guatemala and Honduras, is straining resources and confounding Obama administration security strategies that work effectively in other regions. It is further complicating President Obama’s uphill push on immigration, fueling Republican arguments for more border security before any overhaul.