Some governors are pushing to scale back or kill proposals to expand Medicaid to provide healthcare coverage to the uninsured, raising a new challenge to President Barack Obama's effort to overhaul the system. Medicaid is funded through a combination of federal and state tax money. Proposals in the House and Senate would expand the program to cover at least a third of the nation's 46 million uninsured, but states are worried they would get stuck with a big part of the tab.