WASHINGTON, D.C. – President Bush on Tuesday vetoed a bill protecting doctors from a Medicare pay cut, but both houses of Congress swiftly overrode the veto with large bipartisan majorities, so the bill is now law.

The vote in the House was 383 to 41, with 153 Republicans defying the president. In the Senate, the vote was 70 to 26, with 21 Republicans voting to override.

The bill won more support on Tuesday than when it was first approved. The tally in the House last month was 355 to 59, with 129 Republicans voting for passage. The crucial vote in the Senate was 69 to 30, with 18 Republicans voting yes.

The measure is the fourth bill to be enacted over the president’s veto, and two of those were farm bills.

Mr. Bush has been getting his way on many foreign and national security issues, obtaining money for the Iraq war, persuading Congress to pass new wiretapping legislation and fending off restrictions on harsh interrogation techniques like waterboarding.

But Democrats have gained the upper hand on many domestic issues, passing a water projects bill over the president’s veto and forcing the White House to accept new education benefits for veterans who fought in Iraq and Afghanistan.

After experiencing many setbacks on health legislation in recent years, Democrats rejoiced in a resounding victory on Tuesday.

The vote “renews the light of hope for those who need our help the most, senior citizens who depend on Medicare,” said Senator Harry Reid of Nevada, the majority leader.

The House speaker, Nancy Pelosi of California, said: “Seniors’ organizations and disabilities groups support this legislation. Just about every health-care-providing group in our country supports this legislation, except one, and that is some in the health insurance industry. I guess the president is voting with them and not with America’s seniors.”

The political dynamic was illustrated by Representative Marilyn Musgrave of Colorado, a conservative Republican who boasted that she was voting against the wishes of her party. “I am proud to continue my fight against the White House on behalf of Colorado doctors and seniors,” Mrs. Musgrave said. The votes on Tuesday ended a long string of victories for the health insurance industry.

In his veto message, Mr. Bush said he objected to the bill because it would cut federal payments to Medicare Advantage plans and slow the growth of such plans, offered by insurance companies as an alternative to traditional Medicare.

“I support the primary objective of this legislation, to forestall reductions in physician payments,” Mr. Bush said. “Yet taking choices away from seniors to pay physicians is wrong.”

Many independent studies have found that the private plans, sold by insurers like Humana and UnitedHealth, cost the government more per person than traditional Medicare. But Mr. Bush said that reducing payments to the plans would force them to “reduce benefits to millions of seniors.”

The bill also sets strict standards for the marketing of private plans, to curtail high-pressure sales tactics that have prompted complaints from beneficiaries and state insurance regulators.

The main purpose of the bill is to cancel a 10.6 percent cut in Medicare payments to doctors that took effect on July 1.

Little-noticed provisions of the bill would reduce the beneficiary’s co-payment for mental health services and increase assistance to low-income people on Medicare. In addition, the bill would delay a competitive bidding program for suppliers of medical equipment like oxygen tanks and power wheelchairs.

Competition “should be expanded, not diminished,” Mr. Bush said.

The American Medical Association and AARP, the advocacy group for older Americans, lobbied for the bill, deluging members of Congress with messages warning that doctors would be less likely to take Medicare patients if their fees were cut.

Four Republican senators switched sides and voted for the bill on Tuesday. The senators — Christopher S. Bond of Missouri, Richard G. Lugar of Indiana, and Thad Cochran and Roger Wicker of Mississippi — had voted against consideration of the bill on three previous occasions.

Representative Lois Capps, Democrat of California, said the veto showed that Mr. Bush “would rather cozy up to his friends in the insurance industry than improve access to health care for seniors and those with disabilities.”

But Representative Jim McCrery, Republican of Louisiana, said the bill “just kicks the can down the road” and does not fix fundamental flaws in the formula for paying doctors. In 18 months, Mr. McCrery said, doctors will face a 20 percent cut in their Medicare payments.

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