The unlikely alliance between Donald Trump and Sen. Jeff Sessions started because of the United Nations.

It was 2005, and Trump was busy criticizing the U.N.’s plan to launch a $1.2 billion renovation of its Manhattan headquarters. To the real estate mogul, who had constructed Trump World Tower across the street, the price for the remodel was unreasonably high.

“The United Nations is a mess,” Trump told the New York Sun, deriding just the kind of multilateral institution he now routinely pans in his presidential bid. “And they’re spending hundreds of millions of dollars unnecessarily on this project.”

After Sessions learned of Trump’s views, the Alabama Republican and then-Sen. Tom Coburn (R-Okla.) invited him to come to Washington to talk about building renovation and air his criticism of the U.N. project at a Senate subcommittee hearing.

The result was the best congressional testimony Sessions says he had ever heard. Even now, as Trump’s sole Senate endorser and the heart of his presence in Washington, Sessions loves telling the story. That’s partly because he likes to do his Trump impression.

“Y’all are gettin’ taken to the cleaners!” Sessions said while mimicking Trump in a recent interview, his accent drifting somewhere between Queens and the Alabama Gulf Coast. “There is no way it should cost that much! … If you give it to me, I’ll save you a billion dollars!”

Sen. Jeff Sessions imitates Donald Trump telling members of Congress they were being "taken to the cleaners" with the launch of the U.N. headquarters. (Sarah Parnass/The Washington Post)

Eleven years have passed since that hearing, and sitting in his office on Capitol Hill, Sessions can’t suppress his natural affection for Trump. So perhaps it’s no surprise that the ultra-conservative southerner has become Trump’s main man in Washington as the leading presidential campaign careens toward the Republican convention.

“I think he can win, and I believe he will,” Sessions said. “He will need to continue to flesh out the details of his policies. But his instinctive response to Americans’ current situation has been pretty darn good.”

Sessions described why he thinks Trump appeals to a large swath of voters that are seemingly his opposite — neither rich nor well-educated — and why his ostentatious lifestyle and private jet don’t put off supporters.

“I do think it’s one of the charms he has. It’s more of a blue-collar attitude, but he is so proud of that plane!” Sessions said. “He doesn’t try to be cool, like, ‘I’m a rich person.’ He says, ‘Let me show you this, let me show you that!’ He takes you around and he wants to show you his towers.”

The Alabama Republican described Trump’s meeting last month with influential Republicans at the Old Post Office Building in Washington, which Trump is renovating.

“He couldn’t stop telling everybody about what all is going to be in it and how they’re going to have glass here, marble there…. He came to Mobile [Ala.] and my wife and everyone who was on the plane with him, they were charmed. He’s just unassuming and he enjoys people.”

But Sessions is having a hard time selling that Trump charm to his colleagues — no one else in the upper chamber has endorsed the businessman since Sessions on Feb. 28.

The senator serves an increasingly important role for Trump, who will need help from the Washington establishment should he win the White House. Sessions is Trump’s chief resource on policy, his means of introduction to D.C.’s Republican power-brokers and the architect of what some aides describe as a Trump administration-in-waiting.

“I’ve been in two meetings, and in both of them I had to move it along because he really is intellectually curious.… Somebody says something and he latches onto it and they go on and on and on, and other people don’t get a chance to talk. He wants to know more about this, he wants to know more about that, and ask, ask, ask, ask. I think he’ll seek good advice and ask the right questions.”

After the hearing in 2005, Trump and Sessions were out of touch until last June, when they held a conference call on immigration policy and Trump began courting Sessions’s endorsement in earnest. Then Sessions defended Trump late last year when he called for a temporary ban on Muslim immigrants coming to the United States, saying Trump forced an “appropriate” conversation on security.

The Alabamian is best-known for his opposition to the Senate’s 2013 comprehensive immigration bill. But beyond his rejection of both a pathway to citizenship for undocumented immigrants and his support for a wall along the Southern border, he also believes the United States should lower legal immigration. In December, he opposed a Democratic “right-to-migrate” amendment in the Judiciary Committee, which was adopted 16-4.

The affection between him and Trump runs both ways. “Look at him!” Trump said as he watched Sessions bound onstage at a rally in August. “He’s like, 20 years old! Unbelievable guy.”

Now, since Trump said he wants someone who could “walk into the Senate, who’s been friendly with these guys for 25 years” as his vice president, Sessions is the subject of the first wave of Trump VP buzz. (“Don’t bet any money on me,” Sessions recently told reporters.)

A Trump-Sessions ticket would permanently link the political odd couple, with their collision of North and South, brash and mild, business and politics. But the two are already joined by their controversial drive to pull the GOP — and through it, the country — toward nativism on immigration, trade and foreign policy.

“Sessions and Trump are united in the conviction that public policy in the United States should be tailored toward the interests of American citizens,” said Stephen Miller, a longtime Sessions aide who departed for Trump’s campaign in January. “That should be a noncontroversial thought, but it is not in our politics today.”

At 69, Sessions sees the nation-state as the heart of his political mission. Day to day, he leads Trump’s foreign affairs advisory group, courts GOP officials for the Trump campaign and serves as a liaison with organizations such as the Heritage Foundation and the Federalist Society. But, on a deeper level, the Republican senator is thinking about a nationalist revolution.

“The elites have become international, and they’ve ceased to have a primary loyalty to the nation-state,” Sessions said. “Republicans and Democrats do their fundraising cycles, and they go to Manhattan and they have their cocktails, and they hear the whining of some billionaire and ask him for money, and they read the Wall Street Journal, bless its heart — great organization that it is, it’s not perfect. The American people are not Darwinian. We’re not Randian, in a total, brutal survival of the fittest.”

Still, the unknowns of a Trump administration do make the senator nervous.

“Even moderates, they can see in Trump the potential to have logjams broken and things finally get done. This makes some conservatives and some liberals furious, nervous, and me nervous a little bit, because I’m a pretty pure conservative,” Sessions said. “So that’s a potential of his leadership.”

As Trump himself said, the fact Sessions endorsed him over Texas GOP Sen. Ted Cruz in the presidential contest was “big,” not just because of its possible influence in the primary but for how it disappointed the #NeverTrump movement. In public and in private, the conservative commentariat criticized Sessions and grieved losing him to the other side.

“Longtime conservative journalist from Alabama feels betrayed by home state Sen. Sessions. Hard to blame him,” Fox News’s Brit Hume tweeted Feb. 28, the day of the endorsement.

“Trump defends a Mussolini quote, won’t reject the KKK, and *now* Jeff Sessions endorses him?” RedState’s Erick Erickson wrote that day.

“Still hard to believe, Jeff Sessions of all people, endorsed this guy,” National Review editor Rich Lowry tweeted March 4.

Sessions said he has received little blowback from Senate colleagues. Maybe some saw the writing on the wall when Sessions appeared onstage with Trump at a rally in August and, without endorsing him, put on a “Make America Great Again” hat.

But the decision did represent a break for the senator, who many assumed would endorse Cruz, if anyone. Former aides didn’t even see it coming. “He’s normally circumspect about making endorsements, which is what really sort of surprised me,” said Armand DeKeyser, Sessions’s chief of staff from 1997 to 2005.

Though Sessions is all-in for Trump, he hasn’t managed to persuade any of his colleagues to make the same leap. His outreach on the campaign’s behalf has been informal, and he has not been tasked with netting further endorsements. But Sessions said that privately, his colleagues are more receptive to Trump’s views than they tell the press.

As for those Republicans who might suspect Trump’s moral character — his marital infidelity and two divorces have been widely discussed — Sessions points them to a biblical story of a pagan king who released the Jews from captivity and helped them rebuild the temple in Jerusalem.

“Scripture says, ‘He didn’t know the Lord, didn’t respect the Lord, but the Lord used him to advance his kingdom,'” Sessions said. “I just believe that at this point in history, Trump will defend religious faith. I talk to him about that.”

Sessions is familiar with controversy, racial and otherwise. In 1986, President Ronald Reagan nominated him for a federal judgeship. The ensuing confirmation process turned ugly over episodes from his past: the times he called the NAACP “un-American,” prosecuted civil rights activists for voter fraud and said, according to a former colleague, that he used to think the Ku Klux Klan was “okay.”

As for Trump’s controversial remarks, Sessions said he believes he’ll tone it down.

“I think he has said that himself,” Sessions told reporters this month off the Senate floor. “He’s made some mistakes. I think people have been willing to forgive him.”