When does a president usually experience a reduction of political power?

Presidents whose party controls Congress have the most power. If the President's party loses control of either house due to mid-term elections, or even if they lose several seats, the President's power is reduced.

If the President should push hard for some piece of legislation and fail to get it passed he loses some power. If his veto is over-ridden he loses some clout.

A retiring President loses power after the Presidential election, particularly if his party loses the election.