The Justice Dept is under the preview of the Executive Branch. These corrupt House members won't be able to do shit if they are under investigation.
Except his tweet specifically says the investigations would be through the Senate. He has no authority over what they do. The House Judiciary Committee would almost certainly open their own inquiry, if he ordered a DOJ investigation. It’d just be another round of the same nonsense we saw over the past two years - endless subpoenas of DOJ officials, testimony from people like Wray and Rosenstein, along with threats of contempt and impeachment.