Art Del Cueto wants the wall, but he also needs a paycheck.

Border Patrol agents like Mr. Del Cueto — in the crossfire of the government shutdown fight over President Trump’s demand for a border wall — are preparing to work without pay if the government shuts down Friday night.

“It’s nerve-racking,” said Mr. Del Cueto, who is a vice president of the National Border Patrol Council, a union, and works in Arizona. “It sucks, to be honest.”

Federal bureaucrats have become adept at blunting the effect of shutdowns on federal services, and the two shutdowns this year — a 69-hour interruption in January followed by a nine-hour funding lapse in February — had a negligible impact.

But this month’s bitter and unpredictable fight over Mr. Trump’s insistence that Congress immediately allocate $5 billion for a wall on the Mexican border could be different, with the president on Friday predicting a protracted shutdown unless he got his way.