WASHINGTON -- Republican congressional leaders said they've sealed a tentative agreement on a new prescription drug benefit for the nation's seniors.

It would be the largest expansion in Medicare's history.

Talks that lasted most of the day smoothed over the last remaining wrinkles in the bill. Officials said they include a proposal to have traditional Medicare compete directly with new private insurance plans and a plan to encourage employers to maintain drug coverage for retirees.

The agreement ends months of negotiations over the drug benefit and a broad reworking of the Medicare program.

It must win approval of House and Senate negotiators. The legislation would then go to the full House and Senate, where tough questions await from lawmakers in both parties.

The prescription drug bill has been a top domestic priority for President Bush, who earlier on Saturday used his radio address to make another appeal for a bill.