Assuming a scenario like the
cuban missile crisis timeline.
Alone? No, but i think people here seriously underestimate the effect of the near collapse of civilian logistics and infrastructure that will follow the destruction of several major cities and 10+ million deaths. What remains of the US army (likely a good chunk of it) would have to be repatriated ASAP to put the whole country under martial law.
I think many black communities wouldn't accept the martial law, it's the 60s after all, i can definitively see decently sized insurgencies against the government. This will be a non neglible threat for the federal government.
In this scenario canada is badly hit because of soviet bombers that had to drop their payload because they couldn't reach the CONTUS. Canada can't simply handle several nukes being dropped on its few major cities at the time. And it will be even more badly hit by the famine than the U.S.A. If they don't occupy the whole Canada it could definitively go into chaos.
And all of this is assuming the line of succession is more or less intact, and that no general tries to play the hero. If part of the army splits this will seriously hinder the early pacification and humanitarian effort and could lead to millions more death and slow down the reconstruction.