But if that benefit is tacked on without reform, it will be impossible to fix the program. Once prescription drugs are added to Medicare, the only changes that will be possible are changes that make the system even more expensive.

Today, the Congressional Budget Office estimates that adding a prescription drug benefit to Medicare would cost some $400 billion over the next decade. But that’s the least it could possibly cost.

Some liberals, including Sen. Ted Kennedy, D-Mass., have already said they plan to make the drug benefit more generous once it’s enacted. Look for costs to skyrocket well beyond $400 billion over the next decade. And then imagine what happens when baby boomers start retiring on to Medicare in 2013. Talk about a fiscal train wreck!

Congress has a good model for reform right in front of it. Lawmakers should draw up a new federal program that mirrors the Federal Employees Health Benefit Program, the one that covers them and their staffs.

This program would provide a wide choice of health-care plans for seniors. All those plans would have a prescription drug benefit. The difference would be cost. Seniors who want a health-care plan that covers everything would pay a little more, while those who could live without all the bells and whistles would pay much less. And seniors who wanted to keep their current coverage could do that, too.

If the current Senate bill becomes law, seniors will actually lose options. According to the CBO, at least one third of them will end up losing the health-care plans they have now.

Let’s not race into a mistake we’ll spend decades paying for. It’s time to really reform, not simply expand, Medicare.