I don't have the exact words in front of me, but his speech prior to the mob occupying Congress exhorted them to march to Congress. Given the context of the sedition that happened afterwards, the speech may have crossed the line set by Brandenburg v Ohio of incitement to "imminent lawless action" [1].

I believe during the same speech, he also criticized the Vice President for following his Constitutional obligations instead of (as the President would prefer) taking unilateral decisions to throw the vote to Trump instead.

[1] To be clear, the test in Brandenburg v Ohio is a test of whether or not the government can make the speech outright illegal to speak. This is necessarily a very high bar, and it is much higher in the US than most countries in the world. The bar for whether or not private individuals and organizations should feel compelled to disassociate from an individual is clearly lower. That one can seriously entertain that the President's speech is surpassing the high bar of outright illegal speech should indicate how reasonable it is for others to consider it beyond the pale.