President Obama's job-approval rating has slipped below 50% in the Gallup Tracking Poll for the first time in his presidency, pulled down by concerns about the economy, federal spending and health care legislation.

In daily tracking from Tuesday through Thursday, Obama's approval rating was 49%, down from 50% reported the day before in the rolling three-day average. That's not a statistically significant change, but historically it has some political significance when a president can no longer claim the support of half the American people.

Other national surveys previously have shown Obama below 50%, but the daily tracking survey by Gallup has been particularly closely watched. Obama also had slipped to 49 % in a separate Gallup Poll taken Oct. 1-4.

The bad news for Obama really came during the summer amid growing concern about the economy and unemployment. His approval rating was 60% in the tracking poll at the beginning of July, then significantly dropped that month and the next. Since September, his standing has been holding steadily in the low 50s.

Jeff Jones of Gallup reports that every modern president except John Kennedy dropped below majority approval in the Gallup Poll at some point in their presidencies, and all recovered afterwards to go back above 50%.

Even so, Obama's fall has been relatively fast. Gerald Ford and Bill Clinton slipped below 50% sooner -- Ford in his third month in office, Clinton in his fourth. Obama's experience is close to that of Ronald Reagan, who like Obama dropped below the marker during his tenth month in office. (Reagan beat Obama by a few days.)

Falling fast clearly isn't fatal. Clinton and Reagan, for instance, went on to win second terms.

Among other modern presidents: Harry Truman fell below 50% for the first time at 11 months in office, Jimmy Carter at 13 months, Richard Nixon at 25 months, Lyndon Johnson at 29 months, the elder George Bush at 36 months and the younger George Bush at 37 months.

And Dwight Eisenhower? He went 63 months -- his fifth year in office -- before falling below the marker just once.

The Gallup survey included interviews by landline and cell phone with 1,533 adults. The margin of error is plus or minus 4 percentage points. Gallup has now posted the results on its website.

(Posted by Susan Page and David Jackson)