Which presents the greatest threat to congressional incumbents of the president's party?

A mid-term election after the president’s sixth year in office
Usually, there is always a major realignment with the opponent party winning most of the seats in congress. As a result president often try to push their legislation before this time to avoid the stalemate that come about as a result, and most president often use executive orders to pass through their policies and veto down policies they don't like.

Government taxes ? i guess