LOS INDIOS, Texas—President Donald Trump’s vision of a sea-to-sea wall on the U.S.-Mexico border is running into an unexpected obstacle: opposition from Republicans in Texas, where most of the unfenced land is located.

Many Texas Republicans say a better approach mixes additional fencing and more personnel, as well as better technology to monitor the state’s southern boundary, which is marked by the winding Rio Grande River and in vast stretches by formidable natural barriers.

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