Depends on his choice of target.

A war with Canada would basically be impossible to sell. Even among just Republican voters, support for annexing Canada - peacefully or otherwise - is at 25%. For context, somewhere around 50% of Republican voters are hardcore MAGA. So even his own cult aren't down for a war with Canada. The idea is staggeringly unpopular, and would probably kick off a second American civil war as various blue states, especially those on the Canadian border that would be used as staging grounds for such an operation, decide that being embroiled in a long term guerilla war / insurgency with a trusted ally is a more horrific notion than simply rolling up to the White House and nipping the problem in the bud.

Even dictators need the support of their populace. Putin lives in constant terror of an uprising; he talks to his advisors and close confidants constantly about what happened to Gaddafi. That's his nightmare scenario. Invading Ukraine was about bolstering his popularity by projecting strength and reclaiming what they see as old borders. The idea is generally popular in Russia; they see Ukrainians as "lost Russians" who need to be brought back into the fold.

But Americans see Canadians as Canadians. Those on the left respect us and want us left alone, and those on the right see as a bunch of commie-liberal-socialists who they in no way want as part of their country.

Now, an invasion of Mexico, ostensibly to crush the cartels - packaged as essentially saving the Mexican people from themselves - that would be a much easier pitch. Americans by and large see Mexicans as backwards and primitive, and their country as essentially in thrall to the cartels. This would be like "liberating" Afghanistan and Iraq. Even then, he'd still have an uphill battle; Trump ran as the anti-war candidate and his cult have been high on isolationism for a while. He might be able to sneak it through on the guise of securing the borders, which plays into that isolationist mindset, but it wouldn't take long for mounting casualties and costs to really become a drag on his popularity, even with his voters. American politics goes through cycles of isolationism vs interventionism and they're deep in an isolationist cycle right now, a fact that Trump weaponised very effectively against the Dems in both 2016 and 2024. His promises to secure peace in Ukraine and Gaza were playing strongly into that.

Even a war in Mexico could easily lead to a collapse in consumer confidence; Mexico is America's biggest trading partner and a source of a huge amount of consumer goods as well as cars and so on. All of that trade would be heavily impacted by an invasion. Rising prices in the grocery stores and spiralling national debt as yet another "forever war" tacks on even more costs could easily trigger a recession on their own, and that's without the damage he's already doing.

BTW, the plan of "Run your economy on empty promises, then solve your problems by invading and plundering your neighbours" only worked for the Nazis because currencies were still backed by gold in those days. Canada's main strategic reserve is maple syrup. There's nothing up here to plunder. Yes, Trump has this notion of getting at resources like uranium and rare earth minerals, but those things take time to exploit properly. It's not a short term economic fix. Instead what you've got is 40 million really pissed off people that you now have to pacify at an absolutely colossal expense, and the world's most terrifying insurgency. Remember, Canadians look like Americans, act like Americans, dress like Americans, drive American cars, shoot American guns, talk enough like Americans that it would be hard to tell the difference between "Canadian" and "From another state"... Iraq and Afghanistan would be cake walks compared to an insurgency where the enemy can just drive for a few days and be in any of 48 state capitols, or the national capitol. How do you secure every single government building, every power plant, every factory, every major distribution centre... Hell, if we get pissed off enough, every mall and school (Canadians have something of a history of going much, much further than anyone else when it comes to merciless tactics in war; it's not for nothing that our military calls them "Geneva Suggestions") in a country the size of the continental USA? We'd be talking about the largest domestic security operation the world had ever seen. Iraq and Afghanistan added trillions of dollars to the national debt, and that was just trying to secure two small Middle Eastern nations. A Canadian insurgency would bankrupt the US in a year if they really tried to stop it. And if they didn't we'd be racking up dead congressmen and senators on a monthly basis.