Trump's calculus is always that the counterparty needs him, more than he needs the counterparty.

Yep. But this calculation is incomplete. The U.S. is far more important to Canada than Canada is to the U.S. - but Canada is far more willing to tolerate pain over this disruption than the U.S. is willing to tolerate it. The overall U.S. economy can absorb this kind of rupture with modest dislocation while Canada would experience much larger dislocation - but Canada's far more willing to accept that dislocation than U.S. voters are.

Then again, Trump personally doesn't feel any pain from this at all. He's not running in the midterms. He only somewhat cares whether this has any impact on how his party does - the House is probably already a lost cause, and even though there's important races in border states like Maine and Michigan he probably doesn't care too much about the Senate either.