Frank Newport is the editor in chief of the Gallup Poll. In an LA Times op-ed (free subscription required), he writes that based on what the polling data shows now, things don't look too hot for Bush. The two key points of his analysis are:

Bush's 50% approval rating on job performance: It's lower than that of recent successful candidates during March of election year--including Clinton, Reagan and Nixon.

Point two is the "trial heat factor:" Newport says it's unusual for Bush to be losing to Kerry in the polls during the election year:

Since 1956, of eight presidents who sought a new term, five won. Two of these eventual winners started their reelection years on somewhat shaky ground but quickly recovered. Reagan was tied with Walter Mondale in a Gallup poll survey taken in January 1984. Clinton was behind Bob Dole in two Gallup polls conducted in January 1996. But from February 1984 on, Reagan was ahead of Mondale in every trial-heat ballot that Gallup conducted. And, in similar fashion, Clinton was ahead of Dole in every trial-heat ballot Gallup did from February on in 1996. The other three incumbent presidents who won a new term in the second half of the 20th century — Eisenhower, Johnson and Nixon — never once fell behind their opponents in the election year.

What's the bottom line?

If Bush is reelected, he will become the only president out of the last eight incumbents to win after having been behind a challenger in Gallup polling conducted after January of his election year. And, if his job approval ratings don't rise above 50% in April and May, his reelection would mark the first of those eight to win with less than majority approval in the late spring of their election year.