II:
Also, every impeachment effort has taken place when the party not of the president controlled one or both houses of Congress.
Andrew Johnson was a Democrat or third party, I don't recall, and the Congress was majority Republican.
Richard Nixon was Republican, the US Congress was majority Democratic (although, once it was revealed that Nixon ordered the CIA to stall the FBI investigation into the Watergate apartment complex break-in, Republican congresspeople in droves said they would vote to impeach or convict).
William Clinton was Democratic, and impeached by a Republican House, but failed to be convicted by a Democratic Senate.
The Rule of 67.
Donald Trump better hope that the 2019 US Senate has at least 35 Republicans