The Great Shutdown Showdown of 2019 has shown more clearly than anything before the central reality of Donald Trump’s presidency: He does not lead a country; he leads a movement.

Trump is president of the Republican base. He knows how to make his most passionate supporters vibrate like the reed of a clarinet. And they have the same effect on him.

That is why he was willing to go to the wall over The Wall, despite the fact that most Americans think it is a lousy idea. It is why he proudly claimed ownership of what has now become the longest government shutdown ever, and is deservedly getting more of the blame for the pain that it is causing.

Trump used his first televised Oval Office address not to offer a more persuasive argument, or a fresh compromise, or to bring the country together. The only thing he managed to do in that precious nine minutes was to cheapen our most revered national platform with the same histrionic claims about illegal immigration that he made when he came down the Trump Tower escalator in 2015.

All of this stands in contrast with how presidents normally act — or, perhaps, that should be how normal presidents act — when their promises turn out to be implausible or deeply unpopular.

Amid a backlash to his tax cuts during the blistering 1982 midterm election season, President Ronald Reagan reluctantly negotiated with Democrats to produce a bill that raised government revenue by nearly $100 billion over three years. Facing deficits more severe than he expected when he took office, President Bill Clinton abandoned both the middle-class tax cuts that he had promised during the 1992 campaign and the economic stimulus package that had been his first fiscal-policy proposal. To keep the health insurance companies on board with his health-care legislation, President Barack Obama dropped the Medicare-like “public option” that many on the left had considered a central component of the Affordable Care Act.

In all three cases, those decisions disappointed each president’s most ardent supporters. But each of them went on to be easily reelected.

So why is Trump so intransigent on a wall that was broadly understood, even by a large share of his supporters, to be a pipe dream — or, as Sen. Lindsey O. Graham (R-S.C.) put it, merely “a metaphor for border security”?

Trump, it should never be forgotten, is a president who was put in office with just 46 percent of the popular vote. He won not on breadth of his support, but on its depth in aggrieved pockets of the electorate.

“He believes the immigration issue is the heart of his emotional relationship with his base, and he’s right about this,” notes William A. Galston of the Brookings Institution. “He believes, not entirely incorrectly, that almost anything he would do to expand his appeal would dilute his relationship with his base. He is not going to take any chances with it.”

Still, that is a bad bet for a president who started out historically unpopular, and has pretty much been stuck there since. He would do well to remember that millions who voted for him did it with deep reservations. In bowing to the demands of his strongest supporters — who channel their rage through Rush Limbaugh, Sean Hannity and Ann Coulter — Trump is squandering what’s left of the benefit of the doubt from those who were less enthusiastic.

Those more ambivalent Trump voters bolted during 2018’s midterm elections. Led by suburban women, they handed the House to the Democrats, setting up the standoff that has shut down the government. What should be most worrying to Trump is the shift in the industrial Midwest — in states such as Michigan and Wisconsin — that had been responsible for his narrow electoral college victory two years before.

Surely, even Trump must recognize that what he needs most right now is an off-ramp from the disastrous course that he has set for himself and the country. He needs a capitulation that looks like an ultimatum.

Perhaps that will be a declaration that there is a national emergency along the southern border, which no doubt would bring a court challenge that throws his entire wall-building endeavor into an extended limbo. In the end, such a move most likely would be a meaningless gesture, except for the worrisome precedent it would set, inviting chief executives to abuse the power of their office when their power of persuasion fails to do the job.

Still, Trump’s base, no doubt, will stand and applaud. And that, after all, is the only sound he has ever been able to hear.

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