Forbes chairman Steve Forbes on Monday warned Donald Trump against his trade war with Canada after the president imposed 50% tariffs on $20 billion worth of goods.

Forbes, a Trump supporter, appeared on Fox Business following the collapse of last-ditch negotiations, leading “Varney & Co.” host Stuart Varney to ask Forbes if the U.S. is headed for an “economic war” with its longtime ally.

“Let us hope not, because both countries are very dependent on that trade,” he said. “And the fact of the matter is, in terms of the trade deficit, that comes just from heavy oil, crude, that we export from Canada for our refineries — they can’t use any other kinds of crude.”

Forbes continued, “And so you take out that critical crude, we actually have a surplus with Canada for those that take trade deficits and surpluses so seriously. So I think they’re going to find a way to tamp this thing down, but these threats just create uncertainty.”

He went on to explain that this uncertainty “hurts economic growth and investment.”

Trump on Monday accused Canada of “ripping off” the U.S. and claimed it is “among the worst Nations in the World” to negotiate with, declaring, “WE DON’T NEED CANADA” and threatened 50% tariffs on cars, trucks, automotive parts and steel.

Forbes joined a chorus of critics and explained that “tariff is just another word for tax” and that imposing additional taxes on products won’t bring about the revival of U.S. manufacturing that Trump has been promising — but will result in “less economic activity.”

“Republicans should know that,” Forbes told Varney.

Canada on Tuesday announced its retaliatory tariffs on $20 billion worth of U.S. goods. They are set to start Sept. 8.

Forbes wasn’t the only guest Monday to slam the president’s escalating feud with Canada. Former economic adviser Stephen Moore also had some criticism.

“Do I think now is a good time to fight a trade war with Canada? Probably not,” Moore said Monday, per Mediaite. “Because as I look at the numbers — I was just looking at the numbers ... right before I came on the show — commodity prices are rising right now in the U.S.”

He noted, “That’s a bad sign. That’s an indication that inflation continues to tick up.”

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