House Speaker Paul Ryan said he needs to get to know Donald Trump before he can consider backing the presidential hopeful. Ryan was speaking after Republican leaders' weekly meeting May 11. (Reuters)

Five years ago, Rep. Paul Ryan stood on the House floor, assured of victory. “This is our defining moment,” he said.

On that day in 2011, the House’s new GOP majority approved Ryan’s budget plan — which, in defiance of all political instincts, called for cuts in a government program that voters knew and loved: Medicare. Ryan (R-Wis.), worried about debt, wanted eventually to turn the massive ­health-benefit program over to private insurers.

At the time, one particular Republican objected loudly and publicly. But he was nobody important — just the host of “The Celebrity Apprentice.”

“What he did is political suicide for the Republican Party,” Donald Trump said in an interview recently dug up by Mother Jones.

Today, Ryan — now speaker — still has the House. But Trump, it appears, has the party.

House Speaker Paul Ryan says he cannot support Donald Trump. Trump says he doesn't care. Can the two work out their differences? (Deirdra O'Regan/The Washington Post)

On Thursday, the two men will meet in Washington, striving for party unity after Ryan refused to endorse Trump’s presidential bid. When he arrives, Trump will have nearly clinched the GOP nomination by running squarely against Ryan’s vision of what Republicanism is.

That’s especially clear on the subject of “entitlement” programs such as Medicare. At the time of Ryan’s greatest strength, Trump is turning the party against the very change that Ryan sought power to achieve.

“I’m leaving it the way it is,” Trump said of Medicare in a Fox Business interview this week. “I’m going to bring jobs back to the country. We’re going to make our country rich again.”

The success of Ryan’s ideas — and their gradual transformation into Republican orthodoxy — is a remarkable story in its own right. It began in 2008, with a congressman urging his colleagues to cut taxes big and grab two political live wires at once.

First, Medicare: Many Republicans think the expensive federal system that guarantees unlimited health-care coverage to those 65 and older threatens to bankrupt the nation without spending cuts or significantly higher taxes. Ryan proposed capping the cost by giving seniors a set amount of money to buy their own private insurance. Ryan also proposed changing Social Security to allow younger workers to direct some of their payroll tax contributions to personal investment accounts.

It caught on, at least in Washington. The GOP-led House has now passed five annual “budgets” — theoretical policy statements, not actual changes of the law — that have endorsed a version of Ryan’s Medicare plan.

At the same time, the fractious party failed to agree on other big ideas, like how to replace Obamacare, reform immigration laws and overhaul the tax code. So, by process of elimination, Ryan’s idea became the Republican idea, the best evidence that — in Ryan’s words — the GOP is “a proposition party,” not just an opposition party.

The problem was that voters did not love this proposition.

“I don’t care about my grandkids,” Rep. David Schweikert (R-Ariz.) recalled one voter saying at a town-hall meeting, after Schweikert had explained that entitlements needed to be cut so debt would not overwhelm future generations. “I want every dime,” the man said.

In a 2015 survey by the Kaiser Family Foundation, 70 percent opposed Ryan’s proposals for Medicare.

“How many people have called your office to say, ‘Mr. Schweikert, what is your plan for fixing this?’ ” Rep. Mick Mulvaney (R-S.C.) asked Schweikert on the House floor in February, as part of a back-and-forth about the fact that Ryan’s big ideas did not enjoy broad support.

“I think it is zero,” Schweikert said.

Enter Trump.

“You can’t get rid of Medicare. It’d be a horrible thing to get rid of. It actually works,” Trump said in November. In a campaign where Trump has constantly changed his mind about what he believes, this is a subject where he’s remained constant. Trump agrees with Democratic candidates Hillary Clinton and Bernie Sanders: No cuts to Medicare.

At the same time, Trump rejects Ryan’s entire style of politics, which uses detailed budget projections to sketch out worries for the future, followed by an appeal for shared sacrifice.

Trump’s counterargument lacks such detailed evidence. The evidence is, in effect, Trump himself. He makes little effort to make his numbers add up.

“You have to do the waste, fraud and abuse. There is tremendous waste, fraud and abuse,” Trump said on Fox Business, explaining what he would be able to cut. But Trump’s understanding of the system’s waste has seemed wildly off. In a March debate, Fox News’s Chris Wallace caught Trump promising to save $300 billion from a Medicare program that only spends $78 billion in the first place.

Thursday’s meeting seems unlikely to settle such differences. This week, a friend of Ryan’s told The Washington Post that Ryan would not demand Trump agree to his specific vision for entitlements but rather would search for common ground on broader questions of principle.

“This is a big-tent party,” Ryan said during a news conference Wednesday. “There’s plenty of room for different policy disputes.”

But one of Trump’s campaign advisers suggested Wednesday that Trump might indeed change Social Security and Medicare — but only after he has been in office for a while. “After the administration has been in place, then we will start to take a look at all of the programs, including entitlement programs like Social Security and Medicare,” Sam Clovis said during a public forum, according to a report in the Wall Street Journal.

In the meantime, the trustees who run Medicare have projected that its main trust fund will run short of money in 2030. They have urged lawmakers to pass some sort of measure to prevent that.

That has left backers of Ryan-style change trying to put the best face on a bad situation. Maybe, some say, President Trump could be persuaded to cut a deal — in spite of his campaign promises.

“Maybe he just decides: ‘Hey, Paul Ryan really wants to do this. . . . Maybe there’s a way to make this a winner,’ ” said Rohit Kumar, a longtime aide to Hill Republicans who now works for the firm PwC.

For others, it’s easier to dwell on those who support Ryan’s ideas.

“Basically almost every single person running for the Republican nomination this time would support that position,” Dan Holler, of the activist group Heritage Action for America, said on Wednesday.

He meant that former Florida governor Jeb Bush supported Ryan’s vision. So did Sen. Marco Rubio of Florida and former Louisiana governor Bobby Jindal.

But all those people lost to the candidate who doesn’t.

“Yeah,” Holler said. “When you look at the Republican Party broadly, though, it’s part of Republican Party orthodoxy. And rightly so.”