Ronald Reagan famously said of his early 1960′s switch to the GOP, “I didn’t leave the Democratic Party. It left me.”

This week’s presidential debate was another jarring reminder of how much the Republican Party has changed since Donald Trump took charge in 2016.

Ronald Reagan didn’t leave the GOP. It left him.

It’s impossible to imagine Reagan hedging about a European conflict with the Russians. But there was Trump on the debate stage in Philadelphia, sidestepping questions on whether he wanted Ukraine to win its war with Russia.

“I think it’s in the U.S.’s best interest to get this war finished and just get it done,” Trump said.

Instead of the GOP sending a tough message to the Kremlin and Vladimir Putin, it is now Democrats and Vice President Kamala Harris carrying that banner.

“If Donald Trump were President, Putin would be sitting in Kyiv right now,” Harris said. “Our NATO allies are so thankful that you are no longer president.”

At this summer’s GOP convention, Reagan was almost nowhere to be found. There was no video to celebrate his presidency. No honors, 20 years after his death.

It was almost like he never existed.

It’s an odd choice for Trump’s GOP. Reagan was a towering political figure, who still got under the skin of Democrats many years after leaving office.

But this is about much more than Republicans moving on from Reagan.

Former President George W. Bush is nowhere to be found either — it’s as if he’s in a witness protection program for past GOP leaders.

Bush’s vice president, Dick Cheney — who agrees with Democrats on almost nothing — even endorsed Harris.

“In our nation’s 248-year history, there has never been an individual who is a greater threat to our republic than Donald Trump,” said Cheney, who joined his daughter — former Congresswoman Liz Cheney — in backing Harris.

You can ask what happened to the Cheneys. But the better question is, what happened to the Republican Party?

Why did the GOP shed its Reagan roots as the defender of a free Europe?

Why does the GOP defend rioters who attacked the Capitol on Jan. 6, 2021?

Why would the GOP drop Reagan’s enduring optimism — “It’s Morning in America” — in favor of Trump’s refrain that America is in decline?

Many of those themes have now been picked up by Democrats. They are the cold warriors defending Europe. They talk about optimism, the future and the American dream.

“They left me; I didn’t leave them.”

Reagan could give that speech again in 2024. Except it would be all about how the Republican Party left him behind.

Jamie Dupree has covered national politics and Congress from Washington, D.C., since the Reagan administration. His column appears weekly in The Atlanta Journal-Constitution. For more, check out his Capitol Hill newsletter at http://jamiedupree.substack.com