The National Constitution Law Union (NCLU.ORG) has published it Strategy Memorandum Against Trump Indictments. The theory states the President is presumed to have immunity for the actions he took while in office unless he is convicted of impeachment. Recent US Supreme Court action has reviewed fixed law at the time of the US Constitution. The US President has specific powers in the Executive Branch specified in the original US Constitution and if one strips away later case law and restrictions, the President can argue he has immunity from prosecution.
Read the Strategic Memorandum at the NCLU website.