Impeaching Trump is grossly unconstitutional for four reasons:

1) Trump's speech was protected under the 1st Amendment. SCOTUS's Brandenburg decision is clear:

To be considered incitement, the speech must be directed to inciting or producing imminent lawless action and is likely to incite or produce such action. Abstractions are not enough. You may not like Trump's speech, but he never called for violence. In fact, he called peace.

2) The text of Art. II and Art. II, Sec. 4, make clear: no post facto impeachments. Impeachment is the process of removing a sitting President, not one who has left office. Impeaching a former president is an abuse of congress's impeachment power.

3) Art. I, Sec. 9, prohibits bills of attainder. Congress cannot legislatively determine guilt and inflict punishment
upon an identifiable private citizen. That power belongs to the courts. Trump is now a private citizen. The Senate cannot constitutionally try him.

4) Congress violated due process by handing the president and his legal team no opportunity to present a defense or to formally challenge the article of impeachment.