The past couple of weeks must have been difficult for Donald Trump’s more zealous supporters, what with the president-elect doing a 180 on some of his more extreme campaign initiatives.

Remember all that talk about prosecuting Hillary Clinton? Now it’s, “I don’t want to hurt the Clintons,” as he told The New York Times.

And his vow to authorize waterboarding? That apparently went out the window, too, after Trump had a conversation with the former head of the U.S. Central Command, retired Marine Corps Gen. James Mattis, who told him the technique didn’t work.

So much for a couple of the bigger chunks of red meat Trump threw to audiences during the campaign.

But if the supporters who cheered then think they’re disappointed now, they may not have seen anything yet.

Wait until Trump tries to come through on one of his central promises: to bring back millions of high-paying manufacturing jobs to the U.S.

There is no shortage of economic experts who say it’s a fantasy.

Why?

Because U.S. manufacturers already are producing a lot of goods. They’re just doing it with fewer people due to automation and other technological advancements in manufacturing processes.

As detailed in a new report from the Brookings Institution, inflation-adjusted manufacturing output in the U.S. is higher than it’s ever been, even though employment is down significantly since 1980. The number of manufacturing jobs has ticked up a bit since 2010 thanks to a bump in post-recession demand for cars, but the gains have been modest and slow in coming.

For the reason, we turn to the report, written by Mark Muro.

“Boston Consulting Group reports that it costs barely $8 an hour to use a robot for spot welding in the auto industry, compared to $25 for a worker — and the gap is only going to widen,” Muro wrote. “More generally, the ‘job intensity’ of America’s manufacturing industries — and especially its best-paying advanced ones — is only going to decline. In 1980 it took 25 jobs to generate $1 million in manufacturing output in the U.S. Today it takes five jobs.”

So it’s a fantasy for Trump to suggest that he can dial back the clock to an era when Americans could reasonably expect to walk into a factory with little or no training or postsecondary education and get a well-paying job for life.

What’s not far-fetched on the jobs picture is Trump’s call for a major investment in infrastructure, which would put a lot of blue-collar Americans back to work. Democratic Party leaders have recognized its value and have indicated they would work with him to implement it, but it’s already getting pushback from Republicans who oppose any form of government spending. With the GOP in control of Congress, Trump’s good idea may be smothered by his party’s dogma.

But that’s not to say he still couldn’t improve the jobs picture for the blue-collar, Rust Belt voters who propelled him to his election victory.

As Muro suggested, that would mean helping companies invest in progressive manufacturing technology to ensure the U.S. stays competitive with other nations, and investing in training programs to ensure Americans are prepared to work in modern digital factories.

It also would require Trump to swallow a couple of harsh realities. The first is that a lot of the people whom he promised to put back to work in factories will have to find work in some other field. The government could help them, Muro pointed out, by establishing a national wage-insurance program that would replace a portion of a worker’s lost wages for several years as he or she trained and searched for a job in another field.

The second uncomfortable truth for Trump is that the Affordable Care Act helps those same displaced workers by providing them with insurance subsidies.

The good news is that at least one of Trump’s job-related initiatives — demanding fair trade — could help the manufacturing sector, and leading Democrats have suggested they’re open to discussion on that front.

His supporters might not like to hear this, but the sooner Trump drops his delusional vision of restoring millions of jobs and starts following the advice of experts, the better.

It’s what he did with Mattis on waterboarding, and it’s what he should do on the jobs front as well. In a time of deindustrialization, his jobs promise is the modern equivalent of vowing to put telephone operators and blacksmiths back to work.