They can't impeach. Only for high crimes and misdemeanors.
Already, they are trying to say that Mueller's report leaves the President open to impeachment by Congress, but they are a little retarded and fell for the phrasing in spite of the content.
Since the legal phrasing is "I can't indict a seated President, I don't find him innocent." - they argue that the investigation found something to be guilty of and that there is hope for impeachment.
It would be adorable if it wasn't for the fact they would kill and destroy people with this logic, all with a friendly smile on their face.