Google finds a lot of responses saying it has generally been possible

1. You need to provide evidence that the vehicle meets U.S. federal safety and emissions standards.
2. But safety and emissions standards have generally been harmonized between us and Canada.

So you can currently buy a car in Canada for the us because passing Canadian requires is proof that it satisfies American requirements. Protectionism does not currently matter but you’d need to cover tariffs. Maybe that will remain true, at least temporarily