Construction of the eight miles of wall in Donna had been scheduled to begin in February but did not fully get underway until late October. The project will take months to complete. About 30 miles west of the Donna site, near a state park, crews have cleared land in preparation for more new wall construction that, along with three other sites, constitute the work to be done in Hidalgo County.

Similar preparations are underway across other stretches of the southwestern border.

All told, about 76 miles of replacement wall have been completed along the border, federal officials said, meaning that more than 400 miles must be installed in about 60 weeks to meet the deadline.

Construction executives and those involved in construction of the previous sections of border fence built during the administration of George W. Bush said it was a tough deadline to meet, but was indeed possible.

“I would’ve told you 15 years ago: ‘What are you smoking?’” said Victor Manjarrez Jr., a former Border Patrol sector chief in El Paso who helped oversee the erection of the Bush-era border fence in the late 2000s. “But now, the big difference is it’s not just D.O.D. resources. It’s contractors now who are working a hell of a lot faster than we ever could have imagined.”

Mr. Manjarrez said that during the earlier construction, crews completed “anywhere from a quarter-mile to a third of a mile a day.” Agents overseeing the current South Texas work have been running on short sleep and fielding phone calls at all hours of the day, a scenario familiar to Mr. Manjarrez.

Karl Rove, then a senior presidential adviser, wanted daily updates on the wall’s progress, Mr. Manjarrez said. Mr. Rove, he said, tallied the progress not in miles but in feet, in daily updates dubbed the “Wall Dash board .”