Let us say that the modern era of American politics began in 1965.
Since then, there have been seven midterm elections coming after the election of a new President. Let's consider them.
1966 -- Johnson (D) takes a bath as conservatives rebound from the disaster of 1964. Loses 3 seats in the Senate and 48 in the House.
1970 -- Nixon (R) does okay, gaining a seat in the Senate and losing 12 in the House.
1978 -- Carter (D) loses 3 Senate and 15 House
1982 -- Reagan (R) loses 26 House but none in the Senate
1990 -- Bush (R) very modest losses, one Senate and 8 House
1994 -- Clinton (D) gets brutally hammered, 8 Senate and 54 (!) House.
2002 -- Bush (R) becomes the first President in almost 70 years to make gains in a midterm, picking up 2 Senate and 8 House seats.
Let us say that a Senate seat is worth four points and a House seat, one. (Yes, yes. And a real methodology would include governors and state legislatures as well. Toy model, bear with me.) How do they rank?
Bush II (R) +16
Nixon (R) -8
Bush I (R) -12
Reagan (R) -26
Carter (D) -27
Johnson (D) -60
Clinton (D) -86
Average Republican losses are -7.5, or a Senator and a handful of Representatives. Average Dem losses are -64.3. The best Dem showing is about the same as the worst Republican. Two of the three Dems suffered losses that can fairly be called disastrous, that would impose significant political constraints on the rest of the terms, while three of the four Republicans did just fine.
(By now Carlos' fingers are twitching. Toy model and yes-I-know.) Obviously these are very small sample sizes. And if I had chosen to include the midterms of 1974 (disaster for the Republicans) or 1962 (very modest losses for the Dems), the picture would blur. Still... if Obama were to have a decent midterm, he'd be the first Democratic President since JFK to do so.
Apropos of absolutely nothing, really. Back to writing now.