KANSAS CITY, Mo. — Would you trade health insurance today for tomorrow's Social Security check?

Critics of President Bush's new health insurance and tax proposal say millions of Americans — particularly the poor and the young — could face that decision under the fine print in the plan.

"We don't think there should be a trade-off between a health benefit today and a pension benefit tomorrow," said David Certner, legislative policy director for AARP. "We are already quite concerned."

Under the president's healthcare initiative, the value of your healthcare plan at work would be taxed just like ordinary income, but you would get a big tax deduction first — up to $7,500 for individuals and $15,000 for a family. The deduction would offset the new taxes on your health plan. If you didn't get insurance at work, the extra cash from the deduction could help buy it.

Crucially, though, the deduction would apply not only to income taxes but also to payroll taxes — printed on your check, usually, as FICA taxes — which support Social Security and Medicare.

Including payroll taxes in the deduction is essential to making the plan work, experts said. "The plan would crumble (without payroll taxes)," said Michael Cannon, director of health policy studies for the Cato Institute, which supports the Bush proposal. "One-third of taxpayers wouldn't get any benefit at all if payroll taxes weren't included."

But future Social Security benefits are tied, at least in part, to the payroll taxes a worker pays over a lifetime. That means those taking the payroll deduction to pay for health insurance today would get a smaller Social Security check at retirement.

"People's time horizons are very short," said economist Sheldon Stahl. "This is a very profound departure, and there's been no mention of this in the popular press."

Even supporters of the administration plan concede that the choice between today's health and tomorrow's retirement could happen.

"It is possible," said Matt Moore of the National Center for Policy Analysis, a Dallas-based think tank that supports the health plan. "I think it all depends on how the proposal is implemented."

Supporters insist the trade-off is important for people who lack the money to buy health insurance and don't get it at work. "Social Security benefits don't help you if you're dead," Cannon said.

Certner says there's another concern. Allowing a deduction of payroll taxes today might mean less money available, at least in the short term, for current Social Security recipients, putting the trust fund under additional pressure.

"We don't want anything that would undercut" financing for the old-age retirement fund, he said.

Under Bush's plan, healthcare benefits that exceed the deductions would be subject to both income and payroll taxes. Since so many more people receive benefits under the $7,500 cap than over it, Stahl says, the Social Security trust funds would lose money.

One possible solution, Certner said: trading the tax deduction for a refundable healthcare tax credit.