Portman's departure is the latest to roil the upper ranks of the Bush White House as America gears up for presidential elections in November 2008.

Portman, who is quitting after just over a year, comes only a few weeks after Dan Bartlett, one of the President's closest policy aides, said he was stepping down after almost 14 years at Bush's side.

This is another blow for Bush, whose popularity is sagging due to the unpopular Iraq war, and who has seen a several key figures leave the White House.

Among those who have quit for various reasons are former defense secretary and loyal Bush ally, Donald Rumsfeld, who stepped down after the Democratic party rode a wave of anger over Iraq to victory in congressional polls last year.

Former White House aide Lewis "Scooter" Libby was forced to leave the administration in 2005 amid a probe into a CIA leak case, and former national security staffer J.D. Crouch also left the administration recently.

Bush said Tuesday he had nominated former congressman Jim Nussle as the new White House budget chief following Portman's resignation.

If approved by the Senate, Nussle will have to grapple with an American budget deficit that has widened in recent years.
 
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