WASHINGTON (By Robert Pear, NYTimes) February 12, 2004 - President Bush threatened on Friday to veto any bill that scales back Medicare's prescription drug benefit, which becomes available in January 2006 to millions of elderly and disabled people.

New estimates showing that the benefit will cost $724 billion over a decade have touched off a furor in Congress, where lawmakers originally believed that it would cost much less. Many members of Congress say they want to revisit the Medicare law this year. Conservatives seek cutbacks in benefits and cost controls. Liberals and some centrists want to require the government to negotiate prices with drug manufacturers.

But on Friday, Mr. Bush said, "I signed Medicare reform proudly, and any attempt to limit the choices of our seniors and to take away their prescription drug coverage under Medicare will meet my veto."

White House officials made clear that Mr. Bush would resist any fundamental change in the law, which he described Friday as "a landmark achievement."

The president spoke at a swearing-in ceremony for Michael O. Leavitt, the new secretary of health and human services. Mr. Bush presented himself as a protector of benefits long promised to the elderly, a stance that could work to his advantage in the coming battle over Social Security.

Mr. Bush's comments came three days after new data on the Medicare drug benefit showed costs far exceeding the original estimates by the White House and the Congressional Budget Office.

Members of Congress said on Friday that they intended to re-examine the law and would try to revise it, despite Mr. Bush's veto threat.

Representative Jeff Flake, Republican of Arizona, said he was surprised by the threat. But Mr. Flake said: "I cannot imagine the president would veto a bill that would more closely resemble what he originally wanted, a means-tested benefit to help low-income seniors who cannot afford their medicines. Congress larded up the president's request, and I hope he would not cast his first veto if Congress now acts more fiscally responsible."

Senator Olympia J. Snowe, Republican of Maine, said she would keep pushing legislation to require the government to negotiate prices with drug makers and a separate bill to legalize imports of lower-cost medicines from Canada and other countries.

Senator Ron Wyden, an Oregon Democrat who voted for the Medicare bill, said: "The prescription drug benefit is heading into a danger zone. It cannot stand much more bad news. The combination of rising cost estimates and confusion over the drug discount card is really ominous. In a belt-tightening environment, some members of Congress will want to throw this out and start all over again."

Representative Rahm Emanuel, Democrat of Illinois, said Mr. Bush's comments suggested that "he's nervous." Mr. Emanuel, the chairman of the campaign committee for House Democrats, said, "The president's base, the conservative base of the Republican Party, is upset with a bill that has thrown fiscal restraint out the window."

Many Medicare beneficiaries have said they see the drug benefit as inadequate. Supporters of the 2003 law say they do not want to scale back that benefit at the same time Congress is considering reductions in guaranteed Social Security benefits, as part of Mr. Bush's plan to remake the giant pension program for the elderly.

House Republican leaders and White House officials won passage of the Medicare bill in November 2003 by assuring wavering lawmakers that it would cost no more than $400 billion from 2004 to 2013. In January 2004, the White House put the price tag at $534 billion.

This week the Bush administration said the cost would total $724 billion from 2006 to 2015 - a different period, reflecting increases in enrollment and higher drug prices in later years. The chief Medicare actuary predicts that spending on the drug benefit will be growing 11 percent a year by 2013 and will reach $109 billion in 2015.

Details of the drug benefit are likely to be a subject of fierce debate in Congress until the coverage starts on Jan. 1. Medicare officials worry that the debate, including constant criticism of the law by many Democrats, may discourage elderly people from signing up.

The law authorizes the biggest expansion of Medicare since its creation in 1965. Besides providing a drug benefit, the law creates new managed-care options and increases payments to private insurers as an incentive to enter the Medicare market.

"This law is a landmark achievement in American health care, and millions of older Americans are already benefiting from its reforms," Mr. Bush said, referring to the drug discount card and new coverage for an initial physical examination and screenings for diabetes and high cholesterol.

President Bush has not vetoed any bills. A year ago, in his State of the Union address, he vowed to block changes in the Medicare law, and his words were similar to those used on Friday.

"I signed this measure proudly, and any attempt to limit the choices of our seniors, or to take away their prescription drug coverage under Medicare, will meet my veto," Mr. Bush said in his address to Congress on Jan. 20, 2004.

But the statement did not resonate then as it did on Friday, because it was made at the beginning of an election year, and criticism of the new law was coming from Democrats who wanted to discredit Mr. Bush's record.

Now many Republicans, including Senator Judd Gregg of New Hampshire, the chairman of the Senate Budget Committee, are saying that Congress should pass legislation to ensure that the drug benefit does not cost more than the original estimate of $400 billion.

Scott McClellan, the White House press secretary, said the veto threat was not aimed at a specific proposal. "The president," he said, "was making a general statement," a "very strong statement," that Congress should not tamper with the law.

"The president was making very clear to America's seniors that we stand with you, we made a promise to you, and we're going to keep that promise," Mr. McClellan said. "He's not going to let anybody take away what we have provided to you."

Mr. McClellan accused Democrats of trying to "undermine the reforms that we put in place." But proposals to rethink the drug benefit have also come from influential Republicans.