WASHINGTON -- A key reason that Maine and other states are reluctantly to go along with a controversial federal law to upgrade driver's licenses came into sharp focus Tuesday when U.S. senators questioned the government's cost estimates for implementing the program.

President Bush has proposed spending $200 million to pay for the Real ID program, the 2005 law that requires states to make their driver's licenses more secure and less susceptible to forgery.

But lawmakers from several states, including Maine, believe that amount is insufficient and states would be forced to foot the bill to pay for the law. They estimate that new technology needed to implement Real ID could cost at least $4 billion, and that the law amounts to an unfunded mandate on states.

You're asking us to come in with an enormous investment when it's the federal government's responsibility, Sen. George Voinovich, R-Ohio, told homeland security undersecretary Stewart Baker during a hearing of the Senate Homeland Security and Governmental Affairs subcommittee on Tuesday.

Maine Secretary of State Matt Dunlap said the program could cost the state as much as $100 million over five years to implement. That's worrisome, he said, because Maine's Bureau of Motor Vehicles would become a financial burden on the state.