For the past decade, Sen. Patrick J. Toomey (R-Pa.) has preached the gospel of supply-side economics, winning plenty of debates at Tuesday GOP policy luncheons.

But over time, Toomey has become more and more isolated, a transformation that accelerated during the four years of Donald Trump’s “America First” presidency.

The new generation of senators tends to follow Trump’s populist policies of trade wars and big spending, leaving Toomey with a smaller audience for his catchphrases about “slowing the rate of growth” of entitlement programs that are growing “faster than nominal GDP.”

“A president can always drive an agenda or debate, and President Trump, of course, did that in unusual and unique ways,” said Toomey, who is considering a vote to convict the former president in the upcoming impeachment trial.

Toomey’s decision to retire in 2022 will weaken the tax-cut, free-trade wing of the GOP, one that dates back to Republicans Jack Kemp and Phil Gramm.

Combine Toomey’s looming departure with that of Sen. Rob Portman (R-Ohio), who also recently announced he would retire at the end of 2022, and Senate Republicans are on the verge of a brain drain.

Portman is more of a technocratic realist than Toomey, and the pair have had their share of heated debates. But Senate Minority Leader Mitch McConnell (R-Ky.) has long known if Toomey and Portman got to the same place on tax or health policy, he could probably deliver a bill to sign into law.

Their biggest victory came with the 2017 legislation that slashed taxes by roughly $2 trillion, a package that ballooned deficits but added jet fuel to an economy that started a strong rebound during the Obama administration.

“Very pro-growth, supply-side tax reform,” Toomey said. “It was the classic elements of what we’ve always wanted to do: reduce the number of distortions, lower marginal rates, have a more competitive international system.”

But Trump’s populism has overtaken that traditional conservatism, building a new voting coalition anchored among rural, lower-income voters who blamed free-trade deals for crushing their local economies.

Rep. Tom Reed, a polite moderate from western New York, was one of the first GOP lawmakers to endorse Trump in 2016, because he liked the reality TV star’s approach to the “forgotten man.”

Reed, 49, thinks that is the formula for the Republican future: “The Ronald Reagan Democrats, the fair-trade type of philosophy, the folks that want to stand for U.S. manufacturing and stand up for America first.”

This is not how Toomey, now 59, envisioned the GOP future when he entered Congress in 1999 and befriended a young Paul D. Ryan, who was also in his first House term.

The pair shared a Capitol Hill townhouse for a few years, honing their views that lower taxes and dramatic overhauls to the expensive — but politically popular — entitlement programs would unlock economic potential.

While Ryan rose to House speaker before retiring two years ago, Toomey made an aggressive leap in his third House term, challenging a longtime moderate Republican senator, Arlen Specter (Pa.), in the 2004 primary. Toomey narrowly lost and spent the next four years overseeing the Club for Growth, a political outfit that embraced economic conservatism.

After Specter left the GOP and lost the 2010 Democratic primary, Toomey won the Republican nomination and claimed the Senate seat.

Democrats did all they could to paint Toomey’s strict economic conservatism as favoring rich, corporate interests over workers. But the former banking executive comports himself in a professional manner that softened his image among suburban voters, who appreciated his embrace of some restraints on gun rights.

When Trump was elected, Toomey saw a real path forward for his policy vision. In July 2017, as Republicans pushed to repeal the Affordable Care Act, Toomey led the effort to impose serious spending constraints on Medicaid, previously considered a political “third-rail” proposal.

“We had this big knockdown, drag-out battle, but in the end, the Republican Senate consensus was in fact to do that,” he said.

But the legislation never became law.

Later that year, McConnell deputized Toomey, Portman and two other Republicans to craft the massive tax-cut legislation. With Ryan leading the effort in the House, they delivered the biggest supply-side win since the early Reagan years.

But that early victory soon ran into Trump’s tariff wars with China and other nations, as well as his rewrite of the North American Free Trade Agreement.

The final pieces of the new trade pact were negotiated in House Speaker Nancy Pelosi’s offices, with the California Democrat holding off until she received the seal of approval from the AFL-CIO.

When it came time to vote, nearly every Republican abandoned their free-trade ethos. Toomey was the lone GOP vote against the bill in January 2020.

“We’ve never approached a free-trade agreement with the goal of restricting trade,” Toomey said, rattling off several union-friendly provisions. “It was clear that this was an effort to limit trade.”

Toomey is unapologetic about the declining manufacturing workforce, saying the problem is not offshoring but instead automation inside U.S. plants.

“Not because we’re doing it in Mexico, I’m talking about domestic manufacturing, we did it with fewer workers because we have computers that operate robots,” he said.

He also saw the GOP’s hallmark call for fiscal discipline decline under Trump, who happily signed historically large budgets as long as he got to proclaim it was the biggest Pentagon spending ever.

Eleven months ago, massive bipartisan majorities — including Toomey — approved a series of five bills worth more than $3.6 trillion in emergency pandemic funding that were put on the national credit card.

“When you have a virtually zero-interest-rate environment, the cost of servicing a massive amount of debt is very low. And it kind of disguises the problem that’s building,” he said.

Toomey announced his plan to retire a month before the 2020 election, when Biden got 500,000 more votes in Pennsylvania than Hillary Clinton did in 2016 and Trump’s margins in the rural areas slipped.

Ever since, Toomey has been sharply critical of Trump’s attempts at overturning the 2020 results, the most prominent Pennsylvania Republican attesting to Biden’s victory there.

And, after the Jan. 6 insurrection at the Capitol, Toomey was one of five Senate Republicans to vote against Trump on a procedural motion about the impeachment trial that starts Tuesday.

“Post-election, I think, he had behavioral change. It wasn’t just tweets, the president was actively trying to undermine the results of the election. And that is categorically a very, very different thing,” he said.

Toomey wants Republicans to cast Trump’s behavior aside while appealing to his voters, preaching about the pre-pandemic economy of low unemployment and rising wages.

“This was a very good period for blue-collar workers. And we got there through traditional Republican orthodoxy. It was pro-growth supply-side tax reform,” Toomey said.

It’s just unclear, once Toomey is gone, whether there will be any converts left to take up that economic gospel.