A Donald Trump supporter waits to hear the Republican presidential candidate speak in Costa Mesa, Calif. (Lucy Nicholson/Reuters)

Stephen Stromberg is a member of The Post’s editorial board.

Donald Trump became the presumptive Republican presidential nominee Tuesday night, turning his sights on Hillary Clinton, who, he says, will be easy to beat. She will not be — at least not for him. But Democrats must avoid making a similar mistake, dismissing Trump based on his historically high negative poll numbers without understanding why people are voting for him.

Imagine a Trump supporter. The image conjured up might be a loud white man, middle-age or older, probably “poorly educated” (as Trump has put it), perhaps wearing a white tank top or a shirt with something offensive on it, such as: “My other ride is your girlfriend.”

These caricatures exist. I spotted the man with the “girlfriend” shirt at a Trump rally in Council Bluffs, Iowa, in February, near a child wearing a homemade shirt that read “Made in America” on the front and “Trump” on the back. Trump’s rowdy rallies are more often interrupted by supporters hurling petty epithets at Trump’s opponents (“Lyin’ Ted!”) than they are by protesters. At a rally in Tampa in March, Trump admitted he was about to throw out a group of boisterous supporters because he thought their chants of “Vote Trump! Vote Trump!” sounded like “F--- Trump!”

Yet there are also those like Treston Dunn, a mild-mannered, articulate 21-year-old college student from Fort Myers, Fla., who just got back from a two-year mission trip to Spain. Not “everyone who supports Trump is either a racist or needs a job,” he said. “My goal coming out here was to be able to support Donald Trump, and to show that it’s not just uneducated Americans who support him.”

Though they are undeniably significant elements, bigotry, suspicion and identity-based pandering are not the only forces driving the Trump phenomenon. Nor does Trump’s anti-globalization message explain it fully. If Trump’s appeal were that simple, he would be easier to defeat. In fact, Trump’s broader appeal relies on two factors. First, his voters trust him, a megalomaniacal real-estate maven with a history of abusing the truth. Second, they believe, against plenty of evidence, that he is competent — or at least would be more effective than traditional politicians.

These factors are visible in exit polling. About a quarter of Republicans polled during Indiana’s primary Tuesday said that they most wanted a candidate who “tells it like it is,” and 87 percent of them voted for Trump. About a third said they sought a candidate who “can bring change,” and 65 percent chose Trump. These sorts of numbers have been common during the primary.

Dunn admitted that Trump’s rhetoric is often over the top. “I can’t say I’m in agreement with everything he’s said,” he explained. But he and others excuse the nonsense. Sometimes that means adjusting or elaborating on Trump’s statements, such as his promise to build a border wall and make Mexico pay for it. “Donald Trump has never said he’s going to have Mexico write a check,” Dunn said. “He’s simply going to withhold the money that they owe us on trade.” (Trump subsequently released a different strategy to force Mexico to pay.)

Sometimes Trump’s supporters just assume he won’t do what he says. He’s “an unscripted man,” Dunn said. “It’s Donald Trump being Donald.”

Trump’s ridiculous rhetoric and proposals have functioned as a form of signaling. They project strength, yes, but also, most of all, his willingness to say what is on his mind, which implies that most cherished of political attributes: authenticity.

The result is reasoning like that of Andrea Rancatore, who, at a rally in Manchester, N.H., in February, told me that her candidate is “honest” before insisting that “people need to not take him so literally.”

“Is he going to be able to have peace around the world with these world leaders? Probably not. They already don’t like him. He’s rough around the edges,” said a Trump voter, who did not want to be quoted by name, in Florida in March. “He’s just straightforward [that] this country is run by big corporations and corruption,” the woman said. “It’s good to hear someone being so bold with it all.”

Moreover, she insisted, “he’s good at business.” Practically every Trump voter I have interviewed over the past few months has mentioned his private-sector background. “He’s a great businessman,” Dunn said. “Our country was founded by a lot of great businessmen.”

Inasmuch as there is a theme to Trump’s rhetoric, it is that he is unusually competent, and the towers, golf courses and casinos exist for all to see — the many visible blemishes on his record notwithstanding. For purposes of entertainment, NBC popularized the myth that Trump is an outstanding manager. Even though, starting with his poor ground game in Iowa, his campaign has proved that he pays little attention to crucial details, Trump voters put faith in the mogul image. “He’s a businessman,” Rancatore told me. “He’s going to get the job done.” A key element of this thinking blames typical politicians for not “getting the job done,” which is a more comfortable stance for voters than admitting their expectations are unreasonable.

In many ways, Clinton is ill-suited to respond to Trump’s appeals. Trump offers attitude and emotion, and promises to disrupt the system. Clinton offers 10-point plans and caution, and promises to make the system work better. Trump bets that discipline and organization, which Clinton has, do not matter as much as traditional politicians imagine.

Clinton could bet that enough voters will seek substance that all she must do is present herself as serious. She could also bet that enough voters will view Trump as a con man, as his highly negative poll numbers suggest. But the polls could change. Clinton suffers from voter suspicions that she is dishonest, and Trump has already signaled that he will link her to everything about the status quo that people dislike. He will offer himself as a straight-talking alternative.

Merely calling Trump hateful won’t be enough. Also, because many voters find Clinton disingenuous, it might be hard for her to paint Trump with the same brush. Her best bet is to make Trump appear obnoxious and incapable — and call him on his sensitivity when he overreacts to criticism, which would make him seem brittle as well as over his head. Clinton should make the election about competence.