Political speech is at the core of First Amendment protections on free speech, but there are still legal limits that exist on political speech. (I respectfully disagree with the idea posited by user6726's good answer that political speech is immune from legal restraint.)
Direct incitement to imminent lawless action that is likely to occur can technically be prohibited and people can be arrested for it, but the segments I've seen of the most pro-violence of Donald Trump's rallies have not quite risen to that standard.
He has been couching his incitement-related language carefully; it sounds much tougher than it actually is. "I will pay to defend you if you commit a crime" or even an approving "back in the day, you used to get a punch in the face for X" is not the same as saying "Punch those people in the face on your way out" or "Let's knock those protestors out of here in 3, 2, 1... GO!"
While I'm not saying that a pre-crime promise to cover legal fees can never be enough to rise to the level of conspiracy (that's an interesting question), it also isn't really incitement to riot.
But If Donald Trump, or anyone else, used their political position to speak at a public event and directly incite a riot, then they could be arrested under an applicable law without violating the First Amendment.
There are also other limits on political speech, such as defamation. While a political speaker has incredibly wide leeway, there are still limits that exist, especially if the speaker targets a private citizen rather than another political figure.