It seems to have been accepted without challenge that the President is now a lame duck because his party no longer controls Congress. In reality, of course, this merely restores post-war normality to Washington.
If Presidents who do not control Congress are lame ducks, then Truman, Eisenhower and Clinton were reduced to that status only two years in to their presidencies, and yet managed to achieve re-election nonetheless. Poor Presidents Nixon and Reagan were never able to get their presidencies off the ground, and began as lame ducks from day one.
Perhaps this is why those presidents are universally regarded as being without consequence, especially when compared with the genius of Jimmy Carter, the only President in the past 30 years whose party controlled Congress thoughout his term of office.