Republican presidential candidate Donald Trump speaks before a crowd July 11 in Phoenix. (Ross D. Franklin/AP)

Donald Trump is absolutely certain that he can build an impenetrable wall along the U.S.-Mexico border, in the face of skepticism from experts and federal officials. As proof, the developer cites what he’s doing for the federal government itself.

Along Pennsylvania Avenue in Washington, near the White House, Trump is running ahead of schedule on his plan to transform the historic Old Post Office Pavilion into the — aptly named — Trump International Hotel.

The project has attracted small protests since Trump’s controversial remarks about immigration — he described Mexicans entering the country illegally as “rapists” and “murderers” — and two restaurateurs who had leases in the new building have pulled out.

Protesters lined the curb in front of D.C.'s Old Post Office Pavilion, the future home of Donald Trump's luxury hotel. Some there condemned his comments about Mexican immigrants; others were supporting his the 2016 presidential bid. (Erin Patrick O'Connor/The Washington Post)

[Protesters want Trump’s name removed from D.C. project]

Yet even as she labeled Trump’s comments “idiotic” last week, D.C. Mayor Muriel E. Bowser said the hotel project was important for the city and the revitalization of Pennsylvania Avenue. The Republican presidential candidate vigorously agreed, citing his stewardship of the pavilion as evidence that he can erect a southwest border wall, which has become the signature proposal of his campaign.

“I get things done,” Trump said in an interview with The Post on Thursday. “All you have to do is look down Pennsylvania Avenue and you will see a job that was supposedly very tough and is instead ahead of schedule and on budget.”

The Trump Organization broke ground in Washington two years ago, and construction has in fact stayed well ahead of schedule. The project is being managed by the U.S. General Services Administration.

Along the border, Trump said a wall separating the U.S. and Mexico “is absolutely buildable and can be built for far less cost than people think.” Experts have said a massive structure would cost untold billions of dollars and encounter myriad obstacles, but Trump said he could overcome any problems.

[Experts say Trump’s proposed border wall would be difficult, costly]

“It’s not even a difficult project if you know what you’re doing,” he said. “And nobody knows what they’re doing like I do.”