Washington-area governments would pay a larger share of the cost of completing the Metrorail system under financing formulas being negotiated between congressional leaders and local officials.

For almost 17 years, the federal government has picked up 80 percent of Metro construction costs. Maryland and Virginia, the District and eight local governments have been putting up 20 percent.

Depending on which of several financing formulas under consideration is adopted when lawmakers return next month, local governments could face the prospect of coming up with $135 million to $351 million more than they expected to spend to finish the planned 103-mile Metro system.

As a result, Metro might be forced both to raise fares and delay completion of the Red Line in Montgomery County, Yellow Line in Fairfax and the Green Line in the District and Prince George's.

The Bush administration, led by Transportation Secretary Samuel K. Skinner, opposes continuing the 80 percent federal portion and the tradition of not forcing Metro to compete for federal money with the nation's other transit agencies.

With the threat of a presidential veto -- none of Bush's 13 vetoes as president has been overridden -- Congress is trying to put together a bill that Bush would sign. And that probably means a federal contribution of less than 80 percent, according to congressional and administration sources.

How much of a reduction is unclear. The House has approved legislation trimming the federal share from 80 percent to 75 percent. But some senators want to slash the federal amount further, to as low as two-thirds federal, one-third local. Still another possibility is a 70-30 federal-local split.

"Certainly the House action indicates it would be difficult to sustain the 80-20 match," said Metro board Chairman Mary Margaret Whipple of Arlington. "It's clear from that alone that we won't get 80 percent."

Anything less than 80 percent means it would be up to financially strained state and local governments to pick up more of the $2.7 billion cost of completing the 13.5 miles that are neither built nor financed.

Local governments had planned to put up 20 percent, or $540 million.

But even a 5 percent drop in the federal share, from 80 percent to 75 percent, would mean that the state and local governments would have to cough up an extra $135 million of the construction costs. A reduction to a two-thirds federal share would require $351 million more from the local governments, some of which already have complained about the high costs of supporting Metrorail.

A new request for more local money for construction could result in pressure to raise fares and trim spending, officials said. It could also mean that construction costs would be spread out longer than the 11 years planned for the completion of the remaining 13.5 miles.

"It would be a hardship," said Fairfax County Board of Supervisors Chairman Audrey Moore. "We are having to take on so much more transportation funding without any help from the state or federal government."

The Bush administration and some lawmakers argue that the Washington area, which includes some of the nation's wealthiest counties, can afford a greater share of Metro's costs.

They say that Washington's transit system should be treated more like other systems, some of which are getting more than half their construction money from local governments.

"No other system in the country could hope to be guaranteed the maximum level" of 80 percent federal funding like Metro's, said Rep. Dana Rohrabacher (R-Calif.).

Moreover, other transit systems have to compete for about $400 million a year in federal money for new transit construction, while Metro each year gets its own appropriation. Metro has received $85 million to $250 million a year for the past eight years.

Skinner wants that special status to end, and for Metro to be thrown in with the other transit systems battling over the $400 million. "Metro should go through the same process everyone else does," he has said.

Metro always has been able to convince Congress that it is unique and deserves special funding. The federal government, they say, supported Metro because it wanted a way to move its work force and tourists. Slashing the federal contribution now would be like parents abruptly cutting off their children from support, officials say.

"You can't change the rules in the middle of the game," said Virginia Transportation Secretary John G. Milliken, a former Metro board chairman.

Throwing Metro in with other transit systems competing for new construction money would be like "putting a 500-pound gorilla in the banana grove," in the words of one Senate staff member, because the other transit agencies would have to compete for a relatively small amount of money with an agency that has received over $3 billion in federal money. There's no guarantee the pot would be increased.

Sen. John Heinz (R-Pa.), who must look out for Philadelphia's transit system, also is trying to head off a presidential veto. He has recommended that Metro retain its special status, but that the federal amount be lowered to two-thirds -- the same as it was in Metro's first few years.

A Senate committee approved Metro financing at 80 percent, but few expect that to survive when the full Senate takes up the legislation this fall.

Some elected officials hold out the possibility that the final 13.5 miles will be financed with 80 percent federal money. But as a Senate staff member said, "Getting a $2.16 billion bill through in a year in which we're facing a major budget crunch is risky business. It's inevitable the formula will come down."