David M. Rubenstein, one of Washington’s richest men, has lately been in the business of protecting and restoring many of the country’s historical treasures.

He’s pumped millions of his billions into the Lincoln Memorial, the U.S. Marine Corps War Memorial and Arlington House, which was the home of Robert E. Lee.

Rubenstein even bought — for $21.3 million — a copy of the Magna Carta, which is on loan to the National Archives.

But compared to the plaudits he’s received for those efforts, Rubenstein says nothing comes close to the thanks he’s received for the more than $10 million he has spent helping restore the Washington Monument.

“It’s really an iconic building and probably the most recognizable building in the United States,” Rubenstein said Tuesday morning on the monument’s observation deck. “It’s a statement of American strength.”

The 555-foot marble and granite landmark reopens to the public Thursday after years of repairs stemming from a 2011 earthquake. First lady Melania Trump is expected to attend a ribbon-cutting ceremony and visit the observation deck.

Rubenstein, who made his fortune in private equity at the Carlyle Group, initially donated $7.5 million to repair structural damage to the monument. After it reopened in 2014, the aging elevator repeatedly broke down, often stranding visitors.

Following a second closure in 2016, Rubenstein kicked in an additional $3 million to modernize the elevator.

Visiting the monument this week, Rubenstein recalled visiting as a child while growing up in a working-class neighborhood in Baltimore, where his dad was a mailman.

“My parents brought me over here because the price was right,” Rubenstein said, meaning it was, like nearly all Washington’s other monuments and museums, free.

(Forbes Magazine says he’s now worth $3.2 billion, so presumably cost of admittance to any structure or event on Earth is not an issue.)

It was damage to the monument that got Rubenstein interested in historical philanthropy.

He asked officials at the National Park Service who would fix the monument and how much it would cost. Told that Congress would probably need to be involved, Rubenstein decided to put up the money himself so there wouldn’t be too much delay.

“Then I realized there are other buildings that could probably use some money, so I began fixing up other things,” he said.

Asked what the return on investment was for him, Rubenstein said “infinite.” But that’s obviously not in terms of cold, hard cash.

“Thomas Jefferson said part of what we want to do is pursue happiness,” Rubenstein said. “So what produces the most happiness in the world?”

Having happy and healthy children, he said.

Making your parents proud, he said.

And this: “When you have your fellow citizens think you’re doing something useful, that’s probably pretty good. I get a lot of pleasure out of doing these things, and if I didn’t do them and I died with more money would I be a happier person? I don’t think so.”

Rubenstein moseyed around the observation deck talking to reporters and looking out the windows, which frame some of the best views of Washington — the Capitol, the Lincoln Memorial, the White House.

Then, dressed in a suit and tie, the 70-year-old billionaire prepared to climb a ladder up 19 feet to get a peek at the elevator’s new machinery. A National Park Service official offered some advice: Always have three points of contact on the ladder.

“I can do it,” Rubenstein said.

And off he went.

Descending the ladder a few minutes later, he said, “I don’t think Melania is going to climb up there.”

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