In the survey results out yesterday, the Gallup polling organisation said Obama had previously slipped to 41 per cent several times since taking office in January 2009, but noted his approval rating was 50 per cent as recently as June 7.
The president`s new 40 per cent figure closely tracked the 41 per cent who approved his handling of the angry stalemate in the US Congress over raising the country`s 14.3 trillion USD debt ceiling to avert a disastrous default.
Recent public opinion surveys have found Obama wins significantly higher marks than his top Republican foes or Democratic allies in congress, but Gallup`s daily tracking poll only covers the president.
A recent Gallup survey in early July found Congress had a job approval rating of 18 per cent.
Obama`s approval rating among Democrats was 72 per cent, against 13 per cent of Republicans, and 34 per cent among the unaffiliated voters thought to decide US elections, including the November 2012 contest in which he seeks a second term.
The Gallup survey, which had an error margin of plus or minus three percentage points, also found the US public in an exceptionally glum mood about the fragile economy.
Fifty-two per cent of respondents said economic conditions were poor, the highest since August 2010, while 75 per cent said the economy was getting worse, the highest since March 2009.