WASHINGTON  House Democrats will ask the wealthiest Americans to help pay for overhauling the health care system with a $550 billion income tax increase, the chairman of the tax-writing Ways and Means Committee said Friday.

The proposal calls for a surtax on individuals earning at least $280,000 in adjusted gross income and couples earning more than $350,000, said the chairman, Representative Charles B. Rangel of New York.

It would generate about $550 billion over 10 years to pay about half the cost of the legislation, Mr. Rangel said. As the proposal envisions it, the rest of the cost would be covered by lower spending on Medicare, the government health plan for the elderly, and other health care savings.

With the economy still hobbled and Republicans already sharpening their tax-and-spend attack line, the proposal is perhaps the clearest expression yet of the mandate that Democrats believe they won last November, when voters expanded Democratic majorities in Congress and sent Barack Obama to the White House.