On Capitol Hill, congressional leaders are demanding the Bush administration withdraw a last-minute measure that would severely weaken regulation of workplace exposure to dangerous chemicals. The rule change calls for reexamining how to determine risks posed by workplace toxic exposure. The rule would also impose extra requirements before government officials can impose new limits in the workplace. In a letter to Labor Secretary Elaine Chao, the Democratic chairs of the Senate and House labor committees, Senator Edward M. Kennedy and Congress member George Miller, said the Bush administration is trying to push through the changes before it leaves office.