WASHINGTON — With Congress facing a major shortfall in transportation funding next year, a House bill introduced Wednesday would raise the federal gas tax by 15 cents per gallon to close the gap.

Rep. Earl Blumenauer, D-Ore., announced the proposed increase at a news conference, flanked by an array of labor, construction and business leaders. It would raise the federal tax on gas to 33.4 cents per gallon and on diesel to 42.8 cents.

“Every credible independent report indicates that we are not meeting the demands of our stressed and decaying infrastructure system — roads, bridges and transit,” Blumenauer said.

The tax has not been increased since 1993, and the Highway Trust Fund into which the revenue flows has suffered because the tax has not kept pace with inflation, and better car mileage has reduced fuel consumption.

“Congress hasn’t dealt seriously with the funding issue for 20 years,” Blumenauer said. “With inflation and increased fuel efficiency, especially for some types of vehicles, there is no longer a good relationship between what road users pay and how much they benefit. The average motorist is paying about half as much per mile as they did in 1993.”

The trust fund no longer draws enough money to pay for the nation’s highway and transit bills. Though states levy their own gas taxes — the average combined gas tax bill was 30.4 cents per gallon at the start of the year — they count on Washington for about half of their transportation funding.

To fund the current federal transportation bill, which expires next year, Congress transferred more than $50 billion from general tax revenues. Blumenauer cited estimates that the trust fund will need $15 billion more each year if Congress decides to keep funding at current levels when it considers a new long-term surface transportation bill in 2014.

He said that phasing in a 15-cent per gallon tax over three years would raise about $170 billion in the next decade.

Doing nothing to bolster the trust fund is not a viable option unless Congress is prepared to slash transportation funding, transfer the tax burden to the states or agree to a massive transfer of money from general tax revenues.

The American Society of Civil Engineers concluded last month that a $2.7 trillion investment in transportation and other infrastructure is needed by 2020 if the United States is to remain competitive in the global marketplace. Three years ago, the Federal Highway Administration estimated that more than $70.9 billion worth of repairs were needed to maintain safe infrastructure.

On the Senate side, Barbara Boxer, D-Calif, who chairs the public works committee, has proposed levying a tax on fuel at the wholesale level, collecting money at the refinery rather than the pump.

Blumenauer sought to bolster his bill’s chances with tax-averse House Republicans by displaying a coalition of support at Wednesday’s news conference.

“There’s a broad and persuasive coalition that stands ready to support [this in] Congress,” he said, “including the U.S. Chamber of Commerce, National AFL-CIO, the construction and trucking industry, cyclists, professional groups, numerous associations of small and medium businesses, local governments and transit agencies.”

He pointed out that a gas tax puts the burden on those who use the roadways, “which historically has been acceptable to Republicans, including Ronald Reagan, who increased the gas tax by a nickel a gallon in 1982.”