When a U.S. president is impeached, the House of Representatives has formally accused them of high crimes and misdemeanors via a simple majority vote, acting like a grand jury. The process then shifts to the Senate, which holds a trial to decide whether to remove the president from office. According to Article II, Section 4 of the U.S. Constitution, a president can be impeached for "Treason, Bribery, or other high Crimes and Misdemeanors". This includes abuse of power, violating the public trust, or breaking criminal laws, though "high crimes and misdemeanors" is a broad term not limited to statutory crimes. So, he would have to be convicted, by the Senate, of something like a war crime. I doubt if the sort of crimes that you could call private crimes - fraud, sex offences etc., would be sufficient to remove a president on conviction.
So, the kick Trump right out of the White House, the Dems would have to control both the House of Representatives and Senate, and convict him of something like abuse of power. Abuse of power by a president occurs when the executive uses their authority for improper personal, political, or improper gain, rather than the public good. Examples often include leveraging federal resources for political investigations, obstructing justice, or overstepping constitutional boundaries regarding war powers or executive orders.