President George W. Bush warned Congress on Friday he would veto any legislation that would take away prescription drug coverage for senior citizens, a new Medicare benefit whose rising costs are causing heartburn in Washington.

“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,” Bush said during the swearing-in ceremony for new Health and Human Services Secretary Michael Leavitt.

Rising cost projections for the new Medicare drug benefit have alarmed lawmakers this week and set off renewed calls to restrain spiraling drug prices and allow pharmaceutical imports from Canada.

Facing a projected $724 billion cost over a decade, many lawmakers have urged Congress to change the new Medicare law by granting the government power to negotiate drug prices directly with pharmaceutical companies. The law prohibits such negotiations for Medicare, the government health insurance plan for the elderly.

Bush’s remarks were met by applause, and he did not elaborate on whether any changes to the Medicare plan, which he considers a major achievement of his first term, would be acceptable.

Bush rarely threatens vetoes and has never actually vetoed legislation, preferring instead to negotiate with members of Congress.

“For decades we promised America’s seniors that we can do better, and we finally did. Now we must keep our word,” Bush said.

Republicans in Congress have generally opposed both drug importation and price negotiations. Democrats have supported more spending on drugs for the elderly, but have said the Medicare plan drafted by the Republican-controlled Congress directed too much money to drug companies and insurers, and not enough to needy older Americans.