So Mitt Romney is retiring from the Senate. This is bad news. As excerpts from a forthcoming biography reveal, Romney is cleareyed about what has happened to his party and, if what he says is true, is a profile in courage compared with colleagues who share his horror but are unwilling to say anything.

Yet some of the commentary I’ve seen about Romney comes close to hagiography, which he doesn’t deserve. It’s good to see Romney speaking up now, but the party he’s criticizing is in large part a monster that people like him helped create.

For the basic story of the Republican Party, going back to the 1970s, is this: Advocates of right-wing economic policies, which redistributed income from workers to the wealthy, sought to sell their agenda by exploiting social intolerance and animosity. They had considerable success with this strategy. But eventually the extremists they thought they were using ended up ruling the party.

Before I get into that, let me take on the widespread myth that Romney lost the 2012 election because he was the victim of a smear campaign, and that Democratic nastiness radicalized the G.O.P., paving the way for Donald Trump.

If you remember the 2012 election, which I certainly do, you know that Democrats portrayed Romney as a plutocrat whose policies would hurt ordinary Americans while enriching the wealthy. And this portrayal was … completely true.

In particular, Romney was a strenuous opponent of the Affordable Care Act, a.k.a. Obamacare, which was enacted in 2010 but didn’t take full effect until 2014 — an especially cynical position since Obamacare was very similar to the health reform Romney himself had enacted as governor of Massachusetts. If he had won in 2012, he would almost surely have found a way to block the A.C.A.’s rollout, which in turn would have meant blocking the large reduction in the number of Americans without health insurance after 2014.

But back to the history of the G.O.P. For a generation after World War II (which Donald Trump recently said Joe Biden might lead us into) we were still a nation shaped by the legacy of the New Deal. Under Dwight Eisenhower the top marginal tax rate on the highest-income Americans was 91 percent and roughly a third of American workers were unionized.

And Republicans largely accepted that state of affairs. In a letter to his brother, Eisenhower wrote, “Should any political party attempt to abolish social security, unemployment insurance, and eliminate labor laws and farm programs, you would not hear of that party again”; while there were a few conservatives who thought differently, “their number is negligible and they are stupid.”

Beginning in the 1970s, however, the Republican Party increasingly came to be dominated by people who did want to roll back the New Deal legacy. Frontal assaults on major programs, like George W. Bush’s 2005 attempt to privatize Social Security and Trump’s 2017 attempt to demolish the A.C.A., generally failed, and were rejected by voters — Democrats retook the House in 2018 largely because of the backlash against Trump’s assault on Obamacare. But tax rates at the top came way down, the power of unions was broken, and income inequality soared.

Why didn’t Republicans pay a big political price for their hard right turn? Largely because they were able to offset the unpopularity of their economic policies by harnessing the forces of religious conservatism and social illiberalism — hostility toward nonwhites, L.G.B.T.Q. Americans, immigrants and more. In 2004, for example, Bush made opposition to gay marriage a central theme of his campaign, only to declare after the election that he had a mandate for the aforementioned attempt to privatize Social Security.

Big-money donors attempted a similar play when they poured cash into the DeSantis campaign early this year. It’s doubtful that they shared Ron DeSantis’s obsession with being anti-woke, but they thought (wrongly, it seems) that he could win on social issues and then deliver tax and spending cuts.

But eventually the forces that economic conservatives were trying to use ended up using them. This wasn’t something that suddenly happened with the Trump nomination; people who think that the G.O.P. suddenly changed forget how prevalent crazy conspiracy theories and refusal to acknowledge the legitimacy of Democratic electoral victories already were in the 1990s. The current dominance of MAGA represents a culmination of a process that has been going on for decades.

And for the most part, Republican politicians who probably weren’t extremists themselves went along. For a while this may have been because MAGA was still delivering the right-wing economic goods. Bear in mind that despite all the talk of “populism,” Trump’s main policy achievement was a big cut in corporate taxes. But non-extremist Republicans also, and increasingly, gave in out of fear — for their careers and perhaps even their safety.

It’s to Romney’s credit that he finally reached his limit. But he did so very late in the game — a game that people like him basically started.