Donald Trump said his lunch, a taco bowl from the cafe in his Manhattan building, was his way of celebrating Cinco de Mayo. (Screen shot from Donald Trump's Twitter feed)

Donald Trump marked his first full day as the presumptive Republican nominee for president with an old habit: Tweeting something that people couldn’t believe they’d just seen.

On Twitter and Facebook, the real estate mogul shared a picture of himself smiling widely and eating a sour cream-covered pile of Mexican food inside of a crispy flour shell.

“Happy #CincoDeMayo!” wrote Trump, celebrating, in his way, the Mexican army’s 1862 victory over French invaders. “The best taco bowls are made in Trump Tower Grill. I love Hispanics!”

Within an hour, Trump’s celebration of Hispanic culture and himself had been retweeted more than 34,000 times, rocketing to the top of the Twitter “Moments” metric. By contrast, the White House’s announcement of a Consumer Finance Protection Bureau rule that would widen consumers’ ability to file class-action lawsuits against banks was retweeted less than 150 times. Trump, not the president, had captured the media’s imagination, enjoying Mexican food despite favoring a “deportation force” and a wall on the Mexican border.

“I didn’t predict that the Republican Party would lose its f---ing mind,” tweeted Nate Silver, whose site FiveThirtyEight just published a mea culpa for insisting Trump would not win the nomination.

The Republican presidential front-runner reversed course on a whole load of issues – all on May 4. (Peter Stevenson/The Washington Post)

“We’re sending a squad of fact-checkers to Trump Tower to taste the taco bowl,” tweeted the ­public-service website Politifact.

Reporters quickly pored over the tweet to find meaning. A reporter for Buzzfeed called the Trump Tower Grill to confirm that it did not sell a taco bowl. Milo Yiannopoulous, an “alternative right” writer and speaker who supports Trump, quickly pointed out that the $13.25 item was sold at the Trump Cafe, elsewhere in the mogul’s eponymous Manhattan tower.

Food writers sprang into action, too, pointing to crowdsourced ratings on Yelp that pronounced the food in Trump Tower’s restaurants to be bland. Indeed, the crispy shell that serves as the taco salad’s bowl, and is typically fried in oil or lard, counteracting the health benefits of its contents, was invented by Americans, not Mexicans. In an April poll conducted by Latino Decisions, 79 percent of Latino voters expressed unfavorable opinions of Trump — something he has insisted he will overcome in the general election.

Hillary Clinton’s campaign, which has reacted with something like delight at the prospect of running against Trump, took less than an hour to respond. “ ‘I love Hispanics!’ —Trump, 52 minutes ago,” the campaign tweeted. “ ‘They’re gonna be deported.’ — Trump, yesterday.”

There was no such response from Sen. Bernie Sanders of Vermont. As discussion of the taco bowl swirled, he tweeted that Americans were “tired of working longer hours for lower wages and seeing almost all new income going to the top 1 percent.” It was retweeted less than 1,000 times.