Will Bunch captures the scale of this achievement:
Worse than Richard Nixon in the days before he resigned in disgrace during Watergate, worse than Jimmy Carter during the Iran hostage crisis, much worse than Bill Clinton when he was impeached. Just as Roger Bannister raced through what once seemed the unreachable 4-minute mile, Bush has burst through a barrier once also thought impossible, below the 20-percent mark.
Bunch also lists the low points for each post-WWII president's approval ratings. Only one even comes close (hint: It isn't Nixon).
Run on those coattails, Senator McCain.