Bush is not in the range that Clinton, Reagan, or Nixon were at when they went on to win reelection handily, but he has neither slid down to the level where Carter and Bush41 were at when they lost big. However, his approval does slightly mirror Ford in 1976, where the race in the popular vote went on to be very close. I say this election will stay true to the conventional wisdom, and be a nail biter.
These are the May/ early June approval ratings and horse race numbers for the last 6 presidential elections involving incumbents.
(all info from the Gallup Organization)
June 3-6 2004
Bush Approval: 49%
Kerry 50% Bush 44%
May 28-29 1996
Clinton Approval: 53%
Clinton 47% Dole 30% Perot 16%
June 6-8 1992
Bush Approval: 36%
Perot 35% Bush 30% Clinton 22%
June 1-3 1984
Reagan Approval: 52%
Reagan 50% Mondale 42%
June 10 1980
Carter Approval 32%
Carter 33% Reagan 30% Anderson 21%
June 8 1976
Ford Approval: 45%
Carter 55% Ford 34%
June 13 1972
Nixon Approval: 59%
Nixon 42% McGovern 31% Wallace 19%
(I posted it here a few days ago, but it has since been archived:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=104x1747500)