The Washington Post has a nice summary of how this election compares to other midterms in terms of presidential approval:

With his approval rating just below 40 percent, Bush approaches Election Day less popular than all but two presidents in the post-World War II era. Only Harry S. Truman in 1946 and Richard M. Nixon, who had resigned three months before the 1974 midterms, were lower. Even presidents going into midterm elections with higher approval ratings than Bush’s — Dwight D. Eisenhower in 1958, Lyndon B. Johnson in 1966, Ronald Reagan in 1982 and Bill Clinton in 1994 — have seen their parties suffer major losses.

The best any president has done with an approval rating below 50 percent was in 1978, when Jimmy Carter was at 49 percent. That year, Republicans picked up 15 House seats, exactly the number Democrats need this year to take control of that chamber.