Senator Kent Conrad, who co-authored the measure, told reporters he "has been told by people in the military it might be beneficial" to increase the bounty.
While he doubted the additional money would lure loyalists in bin Laden's "inner circle", Conrad said it might attract those "more distantly connected" to bin Laden, who is thought to be in a region along the Afghanistan-Pakistan border.
The senate vote came just days after Michael Chertoff, the homeland security secretary, said he had a "gut feeling" that the United States was at greater risk of another attack, noting increased al-Qaeda activity and a history of summer attacks.
On Thursday, Bush denied media reports, citing new intelligence assessments, that al-Qaeda is now as great a threat to US soil as in the months before September 11.
Shortly after the 9/11 attacks, Bush said he wanted bin Laden caught, dead or alive. But a year before the 2003 US invasion of Iraq, Bush's emphasis shifted, saying he did not know bin Laden's whereabouts and "I truly am not that concerned about him".