It’s been over three weeks since Donald Trump won the presidential election, but he can’t stop stewing over the narrow result.

“In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally,” he tweeted Sunday.

“Serious voter fraud in Virginia, New Hampshire and California -- so why isn’t the media reporting on this? Serious bias -- big problem!”

Both claims are false.

Nobody, including the Trump aides who were supposed to be watching for fraud, has produced any evidence of illegal votes. And Trump didn’t win the Electoral College in a landslide -- not even close. He won 57 percent of electoral votes, but that’s well below average for winning candidates. His 306 electoral votes are comparable to the 303 John F. Kennedy won in 1960. That kind of election is generally called a squeaker, not a landslide.

Why is Trump raging about the numbers when he ought to be focusing on the job he’s going to have in about seven weeks? He’s worried about two problems: his legitimacy and his mandate.

It’s understandable that he’s annoyed by the recount drive launched by Green Party candidate Jill Stein. It’s not going to change the outcome, as Clinton’s campaign has conceded. Rather, it’s a sadistic bit of political theater that takes one of Trump’s favorite themes, rigged elections, and turns it against him. And it hit a nerve.

As a young builder from Queens, Trump felt excluded from Manhattan. As a real estate mogul, he lashed out against anyone who doubted his claims of success. Now, as president-elect, he can’t shake his anger that anyone is questioning his victory.

But this isn’t just about Trump’s ego. To govern the way he wants, a president needs a mandate. And it’s not clear that Trump has one.

The word “mandate” doesn’t appear in the Constitution, even though presidents have tossed it around for more than a century. So I’ll rely on the late William Safire’s invaluable “Political Dictionary”:

“MANDATE, n: The authority to carry out a program conferred on an elected official; especially strong after a landslide victory.”

Leaving aside Trump’s hot air on the electoral vote, his claim to broad popular support is still weak.

He won about 46 percent of the popular vote; Clinton won 48 percent. You want a mandate? In 1980, Ronald Reagan won 51 percent of the popular vote in a three-way race. In 2008, Barack Obama won 53 percent. In 1932, Franklin D. Roosevelt won 57 percent.

Voters recognize the difference.

A Washington Post poll last week found that only 29 percent of voters think Trump has a mandate to carry out his program; most said they want him to seek compromises with Democrats instead.

By comparison, in 2008, 50 percent said Obama had a mandate. In 2000, 41 percent said George W. Bush, who lost the popular vote, had a mandate.

Trump’s “favorability” has inched upward since the election to 47 percent in the CNN Poll, but he’s still short of a majority. That’s never happened to a president-elect in modern polling before.

As a practical matter, Trump needs a mandate, however defined, to persuade Congress to follow his lead, confirm his nominations and enact his programs — even when Congress is held by his own party.

Republicans hold only 52 seats in the Senate; if only three dissent, the new president will have a problem. And GOP senators are a restive lot; 12 of the 52 did not endorse Trump on Election Day. On Tuesday, in an early show of autonomy, Senate Republican Leader Mitch McConnell of Kentucky said he didn’t agree with Trump over his charges of voter fraud or his demand (in another tweet) to lock up anyone who burns the American flag.

Trump is immune to chastisement; if we learned anything from the campaign, it’s that. But he won’t get away with his bogus claims unless other politicians and voters accede. So it’s not bad sportsmanship for Democrats and others to point out what really happened on Nov. 8; it’s truth-telling, and it matters.

If enough voters remember that Trump didn’t win in a landslide, he’ll have to build a mandate the hard way: by getting things done. If his administration is brilliantly managed and his program succeeds, his percentage of the popular vote won’t matter for long.

For now, he’s on shaky footing -- no matter what he tweets.

By Doyle McManus

Doyle McManus is a columnist for the Los Angeles Times. Readers may email him at doyle.mcmanus@latimes.com. –Ed.

(Tribune Content Agency/Los Angeles Times)