WASHINGTON — Along the 2,000-mile border with Mexico, which sprawls across four states and nine House districts, a single seat is held by a Republican: Representative Will Hurd of Texas.

And Mr. Hurd, a former undercover C.I.A. officer who barely won re-election in Texas’ 23rd District, the largest of the nine, has emerged as perhaps the most persistent critic in his party of President Trump’s wall.

Drawing upon his years undercover and his work in the private sector, Mr. Hurd has a starkly different vision for the southwestern border: fiber optic cables, sensors, radar, drones, increased staffing — but not the concrete or steel barrier that Mr. Trump has demanded before he reopens the government.

He has joined Democrats eight times this month to vote to reopen the government, without wall funding.