The House and Senate early Friday morning separately approved the largest expansion of Medicare since its inception. After six years of debate and compromise, endless politicking and failed votes, seniors in a few years will be able to buy into subsidized prescription-drug insurance. But the passage of the bills hardly means the work is done: Several poison pills, especially in the House version, could complicate the drug plan or make it prohibitively expensive.

For instance, the House version includes a provision that encourages individuals to establish tax-exempt savings accounts to help pay medical bills. The 10-year cost of the provision, according to the Congressional Budget Office, is $174 billion, a figure that will go directly on top of an already record-setting deficit. The House bill further differs from the Senate bill in that it offers lower benefit, has a larger coverage gap for the sickest seniors and does not include a federal safety net if, as is likely in rural areas, private plans turn out to be uncompetitive.

The battle in conference over these will be fierce, and Sen. Olympia Snowe, who has been at the center of the prescription drug debate and has fought for years to get a substantial measure passed, accurately observed the Senate version, “maintains the integrity of Medicare’s traditional ‘fee for service’ program, while still giving seniors the choice of opting for plans provided by private insurers that may better suit their individual needs. Vital to achieving this ‘balance’ between the tested and the new was maintaining an equal benefit no matter which option is selected. We’ve done that and we must preserve these basic principles as we negotiate the Senate- and House-passed bills and when we send a bill to the president’s desk.”

She will find support among Democrats in the House. Rep. Tom Allen, who voted again this proposal and in favor of a Democratic plan, concluded, “Under the guise of providing seniors a prescription drug benefit, House Republicans have instead created a program that would funnel taxpayer money to insurance companies and keep pharmaceutical prices high.”

The Senate bill, which was also supported by Sen. Susan Collins, who added a key generic-drug expansion, is better. But it too depends on seniors being able to pay a yearly $275 deductible, $35-a- month premium and half the cost of the drug. Once a bill passes, seniors should insist that whoever negotiates the prices – public or private – does so aggressively and with an eye on the steep discounts the drug companies accept in other countries. The Senate bill would save seniors an average of $1,200 a year and, as important, provides extensive coverage for those with very high drug bills. Even so, its $400 billion 10-year price tag is less than a quarter of the $1.8 trillion the CBO estimates seniors will spend on prescription drugs during that time.

The Senate has taken a huge step forward on a contentious issue that mingled the ideological divide in Congress over privatization with the clear need for affordable drugs for the elderly. If an affordable bill of substance is to emerge later this year, it must look more like the Senate version than the pale imitation passed in the House.