President Trump may have lost his chief economic adviser over the trade war he’s hatching with China. It looks, though, like he gained one recruit — Charles “Smoot” Schumer.

As in our senior senator and the Democratic leader in the upper chamber. Mr. Schumer this week swung behind President Trump’s tariff initiative.

What a situation. A Republican president is backed up by the Democratic leader in the Senate, while editorials in the New York Times and the Wall Street Journal line up against them.

Mr. Schumer was nicknamed “Smoot Schumer” some years ago for Utah’s Reed Smoot, a notorious protectionist. The Smoot-Hawley tariff act, passed in 1930, helped precipitate the Great Depression.

America spent decades overcoming the consequences of the error. Not until 2016, though, did a president win office on such a pointedly protectionist platform as President Trump’s.

Mr. Trump announced last week that he was going to start with tariffs on steel and aluminum. It humiliated the director of the National Economic Council, Gary Cohn, a free trader. So the ex-No. 2 at Goldman Sachs announced he’s quitting.

Mr. Cohn, a Democrat, wasn’t the only one opposing the tariffs. Some of the savviest Republicans in Mr. Trump’s informal brain trust, like economist Lawrence Kudlow, have long been warning against tariffs.

Smoot Schumer, though, went before the cameras on Tuesday and offered Mr. Trump support. “The president’s instincts to go after China,” he said, “are the right thing to do,” adding that China had taken “terrible advantage of America.”

Mr. Schumer’s backing of Mr. Trump strikes me as newsworthy. Particularly because it wasn’t off the cuff. On the contrary, back in 2004 Mr. Schumer had taken to the op-ed page of the Times to argue for protectionism.

His piece ran under the headline “Second Thoughts on Free Trade.” The piece raised eyebrows partly because Mr. Schumer co-wrote it with a fallen-away veteran of the Reagan White House, Paul Craig Roberts.

Free trade, they complained, hadn’t worked out as they’d hoped. “To call this a ‘jobless recovery’ is inaccurate,” they wrote. “Lots of new jobs are being created, just not here in the United States.”

I once thought it would be the Democrats who’d pick up the theme. Particularly after Mr. Schumer, in 2014, gave at the National Press Club in Washington a speech calling for his party to find a new strategy.

Mr. Schumer reckoned that the Dems had fought the wrong battle by prioritizing ObamaCare, but he didn’t say what strategy he wanted. “At some point,” I predicted, “Senator Schumer is going to come out for protectionism.”

That was in an editorial in The New York Sun, which I edit. It called protectionism “about the last major policy blunder the Democrats haven’t tried in a big way.”

The lingo and ideas that Messrs. Schumer and Roberts used in that piece were echoed in the 2016 campaign, not by the Democrat but by Mr. Trump. Racing through the rust belt, he made the argument work.

That doesn’t mean those of us who favor free trade (as I do) are wrong. It does mean that Mr. Trump has an electoral mandate to do what he promised the voters.

Mr. Trump hasn’t even played his ultimate economic-policy trump card — monetary policy. That would mean a campaign to end the era of floating exchange rates and currency manipulation.

Candidate Trump complained bigly about that problem when he was on the stump. Monetary reform offers a strategic way to level the playing field for China, America and others, without playing whack-a-mole with tariffs.

Once Trump got elected, though, monetary reform was belittled and blocked by, among others, Gary Cohn. Maybe his departure will allow it to be brought to the fore.

It may be too late to stave off a trade war, but the acrimony will be tempered in the White House by the president’s new cheerleader, Smoot Schumer.

This column first appeared in the New York Post.