WASHINGTON (CNN) - The clock is ticking off the seconds to a midnight Monday deadline for an estimated 5 million eligible Americans to enroll in Medicare's prescription drug benefit program or face penalties when they sign up later.

The drug benefit, known as Medicare Part D, subsidizes private health-care plans and requires seniors to choose among a variety of competing company plans. It has been fraught with controversy from the beginning, amid disputes over the program's price and questions about its effectiveness.

Seniors who sign up after the deadline face a 1 percent penalty per month unless they are deemed eligible for a low-income benefit program. President Bush has rejected calls to extend the deadline.

"They needed that deadline or they wouldn't sign up," Health and Human Services Secretary Mike Leavitt said Sunday. "In fact, the actuary for the government told us if we didn't have a deadline, 1.6 million people fewer would actually sign up."

About 37 million of the estimated 42 million seniors eligible for the drug benefit have already signed up, Leavitt said.

He told CNN another enrollment period will open in November, but Monday night represents the last time the lowest rate will be available.

Leavitt said imposing a penalty for those who miss the sign-up deadline is necessary. People cannot be allowed to wait to buy insurance only when they are about to use the benefit, he said.

"It is unfair for people who signed up during the period to pay higher premiums because [other] people put it off," Leavitt said.

Medicare Part D got off to a rocky start in January, when at least 26 states had to step in to reimburse pharmacists for prescriptions for low-income recipients. Leavitt said that half of those not yet enrolled are "low-income eligible ... and they are hard people often to find, so we're continuing to work at that."

Leavitt said there are low-cost plans for those who think they are so healthy they don't need a drug plan. Every state has a plan for less than $20 per month and almost every state has one for less than $10.

Leavitt said the enrollment process need take no more than a half hour, and urged unenrolled seniors to call the Medicare customer service line at 1-800-MEDICARE (1-800-633-4227) with Medicare card and pill bottles in hand.

When CNN called the number Sunday afternoon, a representative came onto the line just a few seconds after an automated system asked initial questions.

A Government Accountability Office report last week found Medicare's customer service representatives gave wrong or incomplete answers to callers about a third of the time, but Medicare Director Mark McClellan told a congressional hearing last week that those problems have been addressed.

Congressional Democrats have called on Bush to drop his insistence on Monday's sign-up deadline and to modify the bill to allow the government to negotiate with drug companies for better prices.

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