There are a number of them. And I'm confident most will get detailed on this thread, But in my mind there is on big one that best represents the first step down the road we are on today.
In the run up to the 1980 presidential election it was George Bush Sr. running vs. Ronald Reagan for the Republican nomination that year. During his campaign, Reagan floated the idea "hey let's just give tax cuts to millionaires (we didn't have Billionaires at the time, that was a direct outcome of the story I'm telling) and pesto-chango the middle class will thrive as a result". That was patently absurd on the face of it, and George Bush, who, at the time was an actual little "c" conservative, rightly called him out on it, calling his plan "voodoo economics". Anyone with a true conservative bone in their bodies knew precisely that Reagan's idea full of shit and solely designed to trick dumb people into letting the rich get richer off the back of the middle class. GEORGE BUSH SR. KNEW THIS, HE SAID AS MUCH. Most other prominent conservatives at the time agreed with Bush Sr.!
Well, long story short, Reagan, due to his nascent populism (not, notably due to his bullshit economic platform), wins the Republican nomination. The precise turning point was when George Bush Sr. let his personal ambition override his moral compass and in order to secure the VP slot, swallowed his dignity and duty to country and started parroting the voodoo economics bullshit. At one point he even went so far as to lie to reporters that he had never made the Voodoo Economics remark (he did, it's on tape).
Three extremely consequential things came of George Bush's moral failure. #1) Tax cuts for the rich became permanently enshrined as Republican orthodoxy. #2) Swallowing and parroting the "Big Lie" for the sake of achieving political power became Republican orthodoxy #3) And perhaps most consequentially, big "C" Conservatives were finally and permanently liberated from the need to be small "c" conservatives.
From there is a straight bright line to the Republican party becoming the party of channeling the tax burden onto the middle class and off of millionaires (and later, billionaires), becoming the party of parroting bald face lies as a party loyalty test as well as a tribal identifier, and ultimately, in it's end stage form, fascism.
EDIT TO ADD: The moral of this story is that it is never the evil doer that gives away the game. There have been evil doers in every age throughout history, and there always will be. They will constantly assail us. Our one true weakness, is the failure of the supposedly principled leaders we depend on... their failure to make that principled stand when it would cost them personally.
It's not the Rush's, the Miller's, or the Trumps. It's the McConnels, the Tillis', the Collin's, the Justice Robert's, the George Bush Sr.'s, etc. etc. ad nauseam, that have failed us.