As congressional leaders from both parties traveled up Pennsylvania Avenue on Thursday for a White House meeting on finances, open hostilities broke out on the traditionally bipartisan House transportation committee over a GOP proposal to cut spending by about $15 billion a year.

Democrats said the plan would cost 490,000 jobs. They called it “mind-boggling” and “fantasy funding”; they said the committee chairman, John L. Mica (R-Fla.), was “out to lunch” and “a dictator.” Alarmed interest groups called the bill a “grave mistake” and a “sure-fire job killer.”

In reality, none of them had seen an actual bill. Although Mica briefed lobbyists and reporters Wednesday and held a two-hour open session Thursday in which he fielded questions from the audience and callers, only the outlines and objectives of the proposal emerged.

Mica’s goal is to match spending to the revenue that flows into the Highway Trust Fund, primarily from the federal gas tax. He said he believes that could provide $35 billion for each of six years, which can be leveraged into $75 billion a year.

“We’ve looked at every way possible to maximize those dollars,” he said.

Mica says the more-than-doubling of the available funds can be achieved by encouraging public-private construction projects, consolidating or eliminating about 70 federal programs, helping fund state infrastructure and streamlining federal requirements to speed completion of projects. Although Mica opposes tolls on existing interstate lanes, he said the bill would allow states to impose tolls on new lanes built on interstate right of ways.

Mica has held hearings throughout the country to sound out the public on transportation needs.

“Across the country and here in Washington, everyone unanimously said ‘You’ve got to do something about the process,’ ” Mica said. “These aren’t our ideas. They came from across the United States. The streamlining does not violate in any way we think our mutual interest in protecting and maintaining the environment.”

The Mica bill is at odds with a legislative proposal expected to emerge in the Senate next week. Senate Democrats favor a two-year transportation plan rather than the traditional six-year blueprint. Mica called a two-year bill a “recipe for bankruptcy of the trust fund.”

“We think we have a pretty good proposal,” Mica said Thursday. “Not that we can’t work with people to make it better.”

The Democrats, who were not in the committee room during Mica’s presentation, responded to the proposal later. They cited two major studies in the past year, one that says an investment of $2 trillion was needed to rebuild roads, bridges, waterlines, sewage systems and dams that are reaching the end of their planned life cycles. Another concludes that it would require spending as much as $262 billion a year.

“Now the majority wants to slash our transportation investment by one-third, giving our competitors an even bigger lead,” said Rep. Nick J. Rahall II (D-W.Va.), the committee’s ranking member. “It’s actually mind-boggling. While our competitors are moving forward, this bill would leave us stuck in a ditch.”

Oregon Democrat Peter A. DeFazio called Mica’s belief that $35 billion could be leveraged into $75 billion “fantasy funding.”

“We need $87 billion just to maintain the existing transit system, just to maintain the existing highways and bridges in this country,” DeFazio said. “If you want to improve it, you’re talking about $115 billion a year, more than three times what they’re proposing.”

Rahall suggested that Congress draw on general-fund money, as it has in recent years, and settle on a two-year bill in the hope that economic recovery would provide additional revenue after that.

Mica said he planned to hold a hearing on the proposal next week.