Many of the country's top environmental groups issued a last-minute appeal to President Clinton yesterday to veto a popular transportation spending bill that contains what they called "unconscionable" provisions restricting the federal government's ability to set mileage standards for new cars and light trucks.

Even as the environmentalists tried to increase pressure on Clinton, they said they were resigned to the fact that it would almost certainly do no good. White House officials said they expect Clinton to sign the $50 billion transportation measure, rather than veto it with a demand that Congress send him a measure free of anti-environmental provisions.

For the past four years, Republicans in Congress have pushed, and Clinton has acceded to, measures preventing the federal government from raising mileage standards or from requiring that light trucks and sport utility vehicles be subject to the same requirements as cars. This year, the environmentalists had been hoping that Vice President Gore's influence would be enough to encourage a stronger White House stand.

"We cannot make strides in fighting global warming if you continue to let Congress tie your hands on this critical policy," said a letter the environmentalists sent to Clinton yesterday.

Among the groups signing it were the Sierra Club, the Environmental Defense Fund, the Natural Resources Defense Council, the National Wildlife Fund and the U.S. Public Interest Research Group.

But faced with what White House officials privately have signaled is Clinton's imminent signature on the bill, environmental groups were deciding where to place blame. Although some environmentalists lately have signaled their dissatisfaction with Gore, who has tried to make environmentalism a signature issue, most of the groups lobbying the White House over mileage standards said they do not hold him accountable for their latest setback.

"The vice president has been honorable and has tried hard . . . to get the president to veto this," said Dan Becker, director of the Sierra Club's global warming program.

The problem, Becker asserted, is with Clinton, who talks passionately about the need to limit "greenhouse emissions," but who does not realize that "jawboning doesn't do it."

"What's been missing is actions that would actually cut global warming," Becker said.

"People are tremendously frustrated, disappointed" by Clinton's likely acquiescence, said Elizabeth Thompson, legislative director for the Environmental Defense Fund.

White House officials, meanwhile, said their soundings on Capitol Hill made it clear that Democrats had no appetite for a veto fight. At a fund-raiser in California last week, Clinton told environmentalists that House Minority Leader Richard A. Gephardt was urging him not to veto the transportation bill, according to a participant who later shared a written account of the conversation with environmental groups.

This account angered many environmental activists. Gene Karpinski, executive director of the U.S. Public Interest Research Group, said Gephardt had assured environmentalists in a meeting Thursday that he would work to ensure that a veto was sustained.

In fact, the account Clinton gave at the fund-raiser seems to have been roughly accurate. Laura Nichols, a Gephardt spokeswoman, said yesterday, "Our counsel to the president was that a veto could not be sustained in the House."