WASHINGTON  Several Democratic senators sharply criticized the secretary of housing and urban development on Wednesday, warning that accusations of wrongdoing threatened to undermine his leadership.

The Justice Department and the housing department’s inspector general are investigating whether the secretary, Alphonso R. Jackson, improperly steered hundreds of thousands of dollars in government contracts to friends in New Orleans and the Virgin Islands.

On Wednesday, Democratic lawmakers also raised concerns about accusations that Mr. Jackson threatened to withdraw federal aid from the director of the Philadelphia Housing Authority after he refused to turn over a $2 million property to a politically connected developer.

The lawmakers pointed to an e-mail exchange in January 2007 between two senior HUD officials who discussed making the Philadelphia housing chief, Carl R. Greene, unhappy by taking away his federal aid.