There would be months or years of prep work, spreading propaganda that Canada was the source of our woes, that they were wronging us. By the time we invaded there’d be just enough “legitimate discourse” about the invasion that the Presidents supporters could claim any effort to stop him was political.
There was a time not long ago where people said you couldn’t do lots of things or you’d get thrown out - then Trump did many of them, even got impeached (twice!) and stayed in office. In practice, these limits are at best inconvenient for a dedicated lunatic.