WASHINGTON — Senator McCain touts his proposal to cut costs for Medicare prescription drug benefits with a line traditionally associated with Democrats: Bill Gates and Warren Buffett, he says, don’t need a government subsidy and can afford to buy medicine on their own.

But Democrats aren’t biting, and instead they plan to argue that Mr. McCain’s call for wealthier Americans to pay higher drug premiums undercuts his chief criticism of his possible opponents this fall: their support for higher taxes on people in the top income brackets.

“We certainly think it’s hypocritical,” a spokesman for the Democratic National Committee, Luis Miranda, said.

President Bush proposed a similar change in last year’s budget, and the Democratic Party chairman, Howard Dean, said it was another example of the presumptive Republican nominee continuing his economic vision. “If you had any doubt McCain represents four more years of Bush, this is it,” Dr. Dean said in a statement, noting that other items in Mr. McCain’s economic plan were “heavily tilted to corporate and wealthy tax giveaways.”

Under Mr. McCain’s plan, retirees voluntarily enrolled in Medicare who earn more than $164,000 annually per couple — $82,000 for singles — would pay higher premiums for prescription drugs. The current average monthly premium for seniors in the program is $35, and people over the threshold would see an increase of between $15 and $80 a month depending on their income, a McCain spokesman, Brian Rogers, said.

Advisers to Mr. McCain say the change would conform the prescription drug portion of Medicare, known as Part D, to the income requirements that already exist for health care coverage in Part B of the government program.

The debate hinges on whether raising premiums for high-income Americans is the same as hiking their taxes. Many economists say there is a clear distinction, particularly since Medicare is a voluntary program, but that line is easily blurred in a political campaign.

In an interview on ABC’s “This Week” on Sunday, Mr. McCain was asked directly if his prescription drug plan was equivalent to a tax increase.

“All it does is say that people are going to pay just as they do in the other parts of Medicare, that can afford to pay. It’s going to be an increase in their payments, sure,” he replied. “Why should we be paying for Warren Buffett or Bill Gates or wealthy Americans who are retired, making $160,000 a couple, not pay[ing] for their own prescription drugs?”

A fellow at the Ethics and Public Policy Center who served as the top budget official for health during Mr. Bush’s first term, James Capretta, praised Mr. McCain’s proposal and rejected the comparison to a tax increase. “There’s a whole world of difference between the two,” he said. “You’re asking people to take a smaller subsidy, not pay a higher tax.”

Mr. McCain is pushing the plan as a way to cut spending for the Medicare prescription drug program, saying it is a part of a “$1 trillion unfunded liability.” He voted against the creation of the program in 2003, citing its cost.

But economists on both sides of the ideological spectrum say the proposal is too narrow to have a major impact on the growing fiscal burden posed by entitlement programs such as Medicare and Social Security.

The campaign estimates that the higher premiums would affect about 1 million Americans and save $2.8 billion over five years, Mr. Rogers said.

“It’s very little money,” a scholar at the Center for American Progress who served as policy director for the John Edwards campaign, James Kvaal, said. “He’s really suggesting a very minor tweak.”

Mr. Rogers acknowledged that the prescription drug proposal alone falls short of needed savings, but he said it was in keeping with Mr. McCain’s “larger commitment to putting these programs on a sounder financial footing for the long term.”

While Democrats have advocated a shift of the tax burden toward wealthier Americans, they have generally opposed efforts to link more Medicare benefits to income levels on the grounds that since all taxpayers contribute to the program, all should be entitled to the same benefits.

A health policy scholar at the conservative American Enterprise Institute, Joseph Antos, said Mr. McCain’s proposal did not amount to a spending cut and sidestepped the broader and more complicated fiscal crisis associated with Medicare. “It’s totally irrelevant to anything having to do with the Medicare program,” Mr. Antos said.