With a crisis looming for transportation funding this year, a bipartisan quartet of U.S. Senate leaders emerged Thursday to say they had outlined a long-term proposal to replace funding that expires Oct. 1.

While the four senators — two Democrats and two Republicans — agreed on the principles that would serve as the bedrock for a new transportation bill, they left to others the critical question that has hamstrung transportation for several years: how to pay for it.

Sen. Thomas R. Carper (D-Del.), chairman of the Transportation and Infrastructure Subcommittee, said the agreement would “enable and encourage our colleagues in the House and Senate to actually be able to do the tough, heavy lifting, that is, to figure out how to pay for the work we really need to do.”

Despite that hole in the doughnut, the agreement is a significant sign of progress in a year when little action had been expected until after the November elections.

“We can act well before our deadline in July or August,” said Sen. David Vitter (R-La.), ranking member of the Environment and Public Works Committee.

That deadline is the projected time that the Highway Trust Fund, which provides the federal portion of highway and transit funding, will be able to meet its obligations. Its primary source of income, the federal gas tax, no longer generates enough money to pay those bills, and there has been resistance to all of the various proposals to bolster or replace it.

If legislation moves forward in the Senate, it will fall to the commerce committee to come up with a funding solution. Funding for the current transportation bill was pieced together from several one-time sources by Max Baucus (D-Mont.), the former Finance Committee chairman who departed the Senate to become ambassador to China.

The effort announced Thursday was led by Sen. Barbara Boxer (D-Calif.), who chairs the Environment and Public Works Committee

“We, as leaders of this committee, have worked across party lines to act before the Highway Trust Fund cannot pay its bills,” Boxer said. “At a time when 70,000 of our nation’s bridges are structurally deficient and less than 50 percent of our roads are in good condition, we must act, and that is what we intend to do.”

The fourth member of the working group, Wyoming Sen. John Barrasso (R), said the agreement called for a long-term bill — “ideally, six years” — of the sort favored by states, which want to know what level of federal funding to anticipate before they launch highway or transit projects that may take several years to complete.

It also would maintain the funding formulas for existing core programs, only allow spending to rise from current levels to keep pace with inflation, and pay special attention to rural areas.

“The principles we have agreed on will ensure that taxpayer money is wisely spent,” Barrasso said.

The House transportation committee’s chairman, Bill Shuster (R-Pa.), also has promised to deliver a transportation bill before the trust fund comes up short. His committee and subcommittees have held several hearings toward that end. The White House also has delivered a proposal.

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