OPINION — Donald Trump’s political problems are almost all rooted in his personality.

The nonstop lying and boasting that have led to a credibility canyon seemingly flow from the president’s fragile ego. His vicious temperament when crossed produces the torrent of below-the-belt Twitter attacks. His apparent inability to trust anyone beyond his immediate family has produced outrages like Jared Kushner’s dubious security clearance. And Trump’s own tough-guy fantasies are probably connected to his hero worship of Vladimir Putin and his avuncular affection for the murderous Kim Jong Un.

But throughout it all, Trump has been slavishly devoted to the MAGA-hatted true believers who elected him president.

Why else has Trump’s wailing over his wall become the centerpiece of his administration?

It is as if this symbolic and probably ineffective border barrier were the equivalent of FDR’s New Deal, John Kennedy’s promise to put a man on the moon and Ronald Reagan’s determination to defeat communism.

Any other president at this moment would be searching for an exit strategy and probably warbling a few bars of Cole Porter’s “Don’t Fence Me In.” But Trump prefers to wall himself into a corner with a border construction plan that is opposed by more than 50 percent of the voters in recent polls by Quinnipiac University and Monmouth University.

After a foolhardy five-week government shutdown, Trump has banked everything on declaring a phony and legally dubious national emergency at the border. Even after the Democratic House repudiated his ersatz emergency, Trump has refused to offer anything to Senate Republicans desperately searching for a way to vote with the president.

A complicated last-gasp strategy to couple the Senate vote on the wall with legislation to curtail future use of national emergencies (presumably by Democratic presidents) fell apart when Trump balked in a phone call with Utah’s Mike Lee.

Unless Mitch McConnell has been breeding rabbits in his magician’s top hat, Trump is headed Thursday for the most humiliating congressional defeat of his presidency. Enough Republicans are likely to join the 47 Democrats to offer a thumbs-down verdict on Trump using his emergency powers to circumvent congressional appropriations and fund the wall.

Trump will be forced to issue his first veto and then (surprise) the fight over the wall will be tied up in the courts for months, probably with construction stalled. Meanwhile, Trump will hurtle from rally to rally, simultaneously assailing Congress and the courts and boasting that the wall has already been built.

[With both parties awash with cash, maybe campaign reform isn’t so quixotic]

But, in stark political terms, there is a glimmer of electoral logic to Trump’s continued pandering to the anti-immigrant zeal of his supporters. Conceivably, Trump’s self-proclaimed image as a world-beater wall-builder might help him again win states like Pennsylvania in 2020.

That is, unless older voters in Rust Belt states become alarmed about the plans in Trump’s new budget to cut Medicare. Willfully handing the Democrats a potent 2020 campaign issue, the Trump budget calls for more than $800 billion in trims to Medicare spending over the next decade.

Part of Trump’s political appeal in 2016 was his explicit break with the austerity budgetary schemes of Paul Ryan and Mitt Romney — green-eyeshade Republicans who talked openly about curtailing Medicare and even Social Security benefits. As Trump said in a 2015 interview, “I’m not going to cut Social Security like every other Republican and I’m not going to cut Medicare or Medicaid.”

Those words are now officially inoperative. Maybe the president could get away with his proposed $1.5 trillion slash in Medicaid spending over the next decade, since programs that aid the poor and the near-poor are not wildly popular in Trump’s America.

But attacking Medicare is about as popular as a national program to confiscate kittens. A poll released in January sponsored by Politico and Harvard’s T.H. Chan School of Public Health found that 88 percent of Americans believe that it is “extremely important” that Congress makes sure that “Medicare benefits are not cut back.”

Trump budget officials might contend that the savings are in the projected growth of Medicare over the next decade rather than actual trims to current spending. But Republicans lost the political argument over the definition of what constitutes a budget reduction during the Reagan years — and these days, as Gertrude Stein might put it, “a cut is a cut is a cut.”

It is telling that a Wednesday petition-drive email from the Democratic Congressional Campaign Committee, signed by Nancy Pelosi, warned about Trump’s $2.37 trillion “cut to YOUR Medicare, Medicaid and Social Security.”

It is bizarre that Trump has risked this political backlash over a budget proposal that has a shelf life of a boring tweet.

The Republicans with their 2017 tax cut gleefully joined the “deficits don’t matter” chorus. As a result, Trump’s own budget document — filled with rosy scenarios and perpetual sunlight assumptions — projects that the national debt will have increased by more than $7 trillion between Trump’s 2017 inauguration and his departure from office in 2025 if (gulp) he’s re-elected.

So why cut Medicare in a phantom budget?

The most plausible guess is that this is what happens when you have a president with the attention span of a 7-year-old at a birthday party with clowns. Budget nerds — faithful to the Ryan-Romney vision of small-government Republicanism — inserted their own parsimonious notions into the Trumpian void.

And now Trump has to pay the political price for a budget document that he probably never read. Sometimes, as parents have warned for decades, there are lasting consequences to watching television instead of doing your homework.