Washington ? Defying a threatened presidential veto, the U.S. House of Representatives on Friday approved a bill to require federal officials to negotiate with drug companies for lower prices for the 23 million senior citizens who have signed up for Medicare’s prescription drug coverage.

Although the bill is unlikely to become law, it is likely to shape the debate that could result in a more limited measure that allows some negotiating between the government and the pharmaceutical industry over drug costs.

Supporters argue that the massive scale of the Medicare program would result in more significant discounts for prescriptions than private insurers are able to obtain.

At the least, the House bill signals a shift in Washington from President Bush’s view that private enterprise and individual citizens should play the most prominent role in the decisions that determine health care costs.

The legislation passed 255 to 170 vote, with 24 Republicans joining 231 Democrats in backing it. All those voting against it were Republicans.

The bill would repeal a ban on letting the government negotiate with manufacturers for lower prices – a provision that was part of the GOP-sponsored 2003 measure that created the prescription drug program. Under the House bill, the secretary of the Health and Human Services department would be empowered to seek the best prices for the prescription drugs used by Medicare participants.

House Majority Leader Steny H. Hoyer, D-Md., hailed the bill as “a very important first step in making prescription drugs more affordable.”

The legislation – one of the measures that House Speaker Nancy Pelosi, D-Calif., promised Democrats would enact within the first 100 working hours after taking control of the House – was strongly opposed by pharmaceutical companies. Several bought full-page ads in newspapers, urging Congress not to tinker with the drug benefit.

The White House has sided with the pharmaceutical industry’s argument and aides have pledged that Bush would veto the bill if it reached his desk.

“We have a Medicare prescription drug reform that has been saving people significant amounts of money; it is effective,” White House press secretary Tony Snow said Friday. “If this bill is presented to the president, he will veto it.”

The bill’s chances in the Senate appear slim.

Senate Finance Chairman Max Baucus, D-Mont., has said he would not embrace the House proposal; instead, he favors a bill that would allow – rather than order – the administration to haggle with pharmaceutical companies over drug prices.

And the committee’s ranking Republican, Chuck Grassley of Iowa, warned that the House bill could “block access to drugs that a senior might need, make it harder to get your medicine at the local pharmacy and result in higher drug prices for younger people.”

But Sen. Olympia Snowe, R-Maine, made clear she plans to push for some version of the House bill.

“Now is the time for the Senate … to harness the buying power of millions of seniors to give them a better value for their health care dollar,” she said.