Health insurers are watching closely as critics of the new Medicare Part D prescription drug program fight to postpone the May 15 registration deadline.

Medicare beneficiaries who fail to sign up for Part D plans by May 15 may have to pay higher rates for Part D coverage in the future.

Sens. Bill Nelson, D-Fla., and Olympia Snowe, R-Maine, are circulating a letter asking the Bush administration and Republican congressional leaders to extend the deadline.

The letter has attracted 48 signatures. The signers include Snowe and 3 other Republicans: Mike DeWine of Ohio, Lincoln Chafee of Rhode Island and Arlen Specter of Pennsylvania.

“By extending the enrollment deadline and delaying late enrollment penalties, we can make sure that our constituents are not forced to make hasty decisions about their health care,” the authors of the letter write.

Critics of the Part D program, including many Democrats, say 10% to 20% of program participants are having serious problems using the program.

The administration already has extended the enrollment period for some low-income people.

Sen. Debbie Stabenow, D-Mich., who has written a bill to extend the deadline, says the letter does not have enough signatures to make Senate Majority Leader William Frist bring the issue up again.

A Senate majority voted in February to extend the deadline but fell 8 votes short of the 60 votes needed to attach the measure to a tax bill.

Participants in a conference call with Stabenow estimated that 15 million of the 43 million Medicare beneficiaries who are eligible for the Part D program have yet to enroll.