It should have happened four years ago, but thank God that Democrats have finally found their way to the decisive argument against Donald Trump: He is a rich, selfish, spoiled upper-class twit who had everything given to him and holds the working-class people who vote for him in contempt.

“Guys like Trump who inherited everything and squandered what they inherited are the people that I’ve always had a problem with, not the people who are busting their neck,” Joe Biden told a CNN town hall held Thursday near his blue-collar hometown of Scranton, Pa.

“I really do view this campaign as a campaign between Scranton and Park Avenue,” the Democratic presidential nominee added. “All he thinks about is the stock market.

Biden’s move no doubt came naturally to a man for whom Scranton has long been a touchstone of self-definition. But it also reflected a decisive strategic move: The former vice president, having already built a lead by rallying Black voters and well-educated Whites, is now encroaching on Trump’s last bastion.

To the extent that Trump is competitive in swing states, it is because he is holding on to the bulk of the support he won four years ago from Whites without college degrees. If Biden can peel away just 10 or 15 percent of these voters, he will win going away.

And the 2018 elections showed that this is entirely within Biden’s reach. In Pennsylvania, Michigan and Wisconsin, Democrats won statewide races for governor and the U.S. Senate not only by boosting turnout among Trump critics but also, according to exit polls, by winning over a comparable share of Trump’s 2016 electorate.

And we haven’t yet gotten to Biden’s most effective salvo: “When you guys started talking on television about, ‘Biden if he wins, would become the first person without an Ivy League degree to be elected president,’ ” I’m thinking, ‘Who the hell makes you think I have to have an Ivy League degree to be president.’ ”

Bingo.

Anti-elitism is a very American, small-d democratic disposition. For decades, conservative supporters of the economically privileged have cleverly shifted the grounds for deciding the “who is an elitist?” question.

It’s a tactic that has worked in a lot of GOP presidential campaigns, notably 2004. Rather amazingly, the campaign of an incumbent who was a president’s son, George W. Bush (Yale ’68, Harvard Business School ’75), sold a lot of the country on the idea that he was the anti-elitist and that John F. Kerry (Yale ’66, Boston College Law School ’76) was the elitist in the race. (Yeah, it sure looked like a draw in the elitism Olympics to me, too.)

Pay no attention, the right-wing propagandists say, to the fact that contemporary conservatism is dedicated to enhancing the incomes of economic elites through policies such as Trump’s investor-friendly $2 trillion tax cut. No, they insist, the real elitists are those snobby Ivy League do-gooders who look down their noses at mill hands and machinists, construction workers and coal miners.

Let’s concede that, too often, educated liberals have played right into this propaganda by placing a far higher value on academic achievement than on other measures of human worth. As the philosopher — ironically, perhaps, the Harvard philosopher — Michael Sandel argues in his new book, “The Tyranny of Merit,” credentialism is “the last acceptable prejudice.”

Biden has never been guilty of this. I have heard him talk with real passion over the years about how Democrats just don’t understand the people he grew up with and don’t know how to talk with them. In his dialogue with the voters CNN gathered in Moosic, Pa., he got a chance to show how it’s done.

Why did it take so long for Democrats to make clear that Trump is a Park Avenue Plutocrat, not a Paragon of Populism? Partly because they have been intimidated by Republicans who claimed that those who dare to criticize class inequality are engaged in some shameful sort of “class warfare” and a “politics of envy.”

This, of course, ignores the fact that it’s workers — because of structural changes in the economy, tax laws advantaging the wealthy and union-wrecking government policies — who have been the victims of a class war waged by the better-off and their political allies. And “envy” has always been a bit of verbal jujitsu deployed to push aside another word, “greed.”

The state-school striver from Scranton has decided he doesn’t have to worry about any of that. Biden needs only to concentrate on the facts about who Trump is: a pampered elitist and fortunate son who loves golf a whole lot more than he loves working people. I can’t wait to see what lies Trump will tell to hide this truth.

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