Fighting words are legal no-nos. Just about everything else is protected by the freedom-of-speech clause.
At his presidential inaugural, Barack Obama said “those who cling to power through corruption and deceit and the silencing of dissent, know that you are on the wrong side of history; but that we will extend a hand if you are willing to unclench your fist.”
For those who want it quick, it is at around 3:56 of part 2 of his speech:
And who can distrust a man who can run with a dog?