The Department of Education is preparing to exercise broad new powers in the coming weeks that could fundamentally recast how millions of students pay for college. This initiative could transform the federal government from a guarantor of student loans into the dominant provider, replacing the outside lenders to whom students and their families have long turned. Though the Education Department is aiming to dramatically expand its role as a direct lender to fill the void created by an exodus of private-sector lenders, due primarily to the credit crisis.