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Reuters.com, By Donna Smith and Richard Cowan

WASHINGTON, July 15 (Reuters) - In what likely is the last big showdown between President George W. Bush and congressional Democrats over the popular Medicare health care program, the U.S. Congress on Tuesday voted to override his veto of a bill to keep doctors' payments from being slashed.

By enacting the measure over Bush's objections, Congress rescinded an 11 percent reduction in government payments to doctors treating elderly Medicare patients.

Just hours after Bush vetoed the legislation, the Senate voted 70-26 to overturn him, following the House of Representatives, which voted 383-41 to override. The bill now becomes law.

Twenty-one Republicans in the Senate and 153 in the House broke ranks with Bush and joined majority Democrats to overturn the veto in this election-year vote.

Supporters of the legislation argued that the scheduled 11 percent pay cut for doctors would discourage many of them from taking on Medicare patients.

The bill would offset the cost to the government of restoring the doctors' pay by cutting payments to big insurers, such as UnitedHealth Group Inc (UNH.N: Quote, Profile, Research, Stock Buzz) and Aetna Inc (AET.N: Quote, Profile, Research, Stock Buzz), which have contracts with the Medicare program.

Democrats argued that those contracts with private health care plans, which were encouraged in the 2003 legislation creating a new government drug benefit for the elderly, cost more than providing health coverage under the traditional Medicare program.

They also argued that more generous subsidies to private health plans threaten to undermine the traditional Medicare program.

MESSAGE TO BUSH

"Let's send a message to the president his days of doing us harm are very, very limited," said House Ways and Means Committee Chairman Charles Rangel, a New York Democrat.

Over the years, Democrats and Bush have clashed over his proposed budget cuts for Medicare and the huge new prescription drug benefit he pushed through Congress in 2003.

Tuesday's votes marked the fourth time in his two terms that Bush has had a veto overturned by Congress. Bush has vetoed 12 bills during nearly eight years as president. Nearly all of those vetoes were since Democrats gained their congressional majority in 2007.

Bush said the $13 billion in reimbursement cuts to insurers will discourage program participation and reduce choices for the elderly.

"I support the primary objective of this legislation, to forestall reductions in physician payments. Yet taking choices away from seniors to pay physicians is wrong. This bill is objectionable, and I am vetoing it," Bush said in a statement to the House.

Doctors and the seniors' group AARP supported the bill and waged an aggressive lobbying effort to prevent the doctors' pay cut. The legislation was approved by Congress last week with strong bipartisan support.

The AARP issued a statement on Tuesday saying it will make sure its 39 million members get information on how lawmakers voted on the veto override.

"This bill will improve Medicare for the 44 million Americans who depend on it for quality, affordable health care," said Nancy LeaMond, AARP's executive vice president.

The bill is a temporary measure designed to stop the pay cut for doctors and give Congress and the next president, who takes office Jan. 20, 2009, time to review broader issues surrounding Medicare. The health care program faces growing financial strains as the 77 million baby boom generation retires and begins to draw on government benefits. (Additional reporting by Tabassum Zakaria; Editing by David Alexander)