President Bush threatened on Friday to veto any bill seen as weakening a new drug benefit for Medicare patients, after cost projections far exceeding earlier White House estimates led lawmakers to urge changes.

The White House turned up Bush’s rhetoric by accusing Democrats of trying to undermine the drug benefit in the old-age health plan, and it reiterated opposition to letting the government negotiate prices with pharmaceutical companies.

The Senate’s top Democrat shot back that Bush’s threat sought to protect “only special interests” — big drug companies and health insurers that Democrats say unduly benefit under the new law.

Speaking at a swearing-in ceremony for Health and Human Services Secretary Michael Leavitt, Bush said he would veto “any attempt to limit the choices of our seniors and to take away their prescription drug coverage under Medicare.”

Bush’s Republican party controls both houses of Congress, and he has never vetoed a bill.

However, new projections putting the cost of the Medicare drug benefit at $724 billion for the 10 years starting in 2006 have sparked concern among Democrats and Republicans on Capitol Hill, who are worried about this year’s record federal budget deficit.

The new estimate for the drug benefit was far higher than the $400 billion figure, for 2004-2013, that White House gave Congress before the Medicare legislation was passed in 2003.

To restrain the costs, Democrats and some Republican allies are pushing to let U.S. health officials negotiate prices directly with pharmaceutical companies. The Medicare drug law bans such negotiations.

Some House and Senate members also are promoting legislation that would let Americans buy less-expensive drugs from Canada.

“I think that you’ve seen some Democrats, in recent weeks, talking about undermining these reforms,” White House spokesman Scott McClellan told a news briefing.

The president did not elaborate on whether any changes would be acceptable to the Medicare plan, which he considers a major achievement of his first term.

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

Bush’s veto warning was not aimed at specific lawmakers or proposals, McClellan said. But he reiterated Bush’s opposition to lifting the ban on price talks with drug companies and said the proposal would not save money.

Senate Democratic leader Harry Reid of Nevada accused Bush of catering to special interests. “This is an attempt by the president to stop the bipartisan groundswell for drug reimportation and price negotiation, and just the latest example of the Republican party putting special interests ahead of the American people,” he said.