WASHINGTON — The U.S. House is expected to vote as early as Tuesday on a bill to shore up the Highway Trust Fund, the nearly dry federal account that pays a quarter of the cost of highway and bridge projects in Washington state and around the country.

But neither the stopgap House bill nor a competing measure from the Senate tackles the main cause of the highway fund’s chronic shortfall — the federal gas tax that has remained unchanged since 1993.

Instead, Congress is proposing to keep the fund solvent for eight more months or so by cobbling together more than $10 billion through a variety of unrelated bookkeeping maneuvers. Tactics include temporarily boosting revenue by allowing employers to defer tax-deductible pension contributions, going after doctors delinquent on their taxes and tapping money earmarked for cleaning up leaking underground petroleum-storage tanks.

U.S. Transportation Secretary Anthony Foxx has warned states his agency will begin conserving cash starting Aug. 1 by prorating payments and halting same-day reimbursements.

For Washington, that means it would get only about 72 percent of requested highway funds, said Lars Erickson, spokesman for the Washington State Department of Transportation (WSDOT).

The state in fiscal 2014 was reimbursed $875 million from the federal account.

To prepare, WSDOT has identified 39 road projects that could be delayed or stopped if federal money is cut back. They include replacing anchor cables on the eastbound Interstate 90 floating bridge across Lake Washington and paving a stretch of West Valley Highway from South 180th Street to Southcenter Boulevard in Tukwila.

In addition, a federal money shortfall could affect maintenance work on five state ferries scheduled between August and December. WSDOT has not had to implement any of its contingency plans.

The authorization for the Highway Trust Fund expires Sept. 30. But the fund’s balance could hit zero before then. Lawmakers, who leave town Aug. 1 for their five-week recess, are under pressure to reach an agreement quickly.

The crux of the problem is that the federal gas tax has failed to keep pace with inflation, said Nick Nigro, senior manager for transportation initiatives with the Center for Climate and Energy Solutions, a Washington, D.C., nonprofit that works to curb greenhouse gases.

The fund was created in 1956 on the principle that users of the interstate highway system should largely pay for its construction and upkeep. But Congress has left the gas tax at 18.4 cents a gallon since 1993, when a gallon of gas cost little more than a buck.

Had the tax kept pace with inflation, it would be about 29 cents per gallon today, according to the congressional Budget Office (CBO).

A penny increase in the federal gas tax is worth about $1.5 billion a year.

For the current fiscal year, the CBO estimates the highway account will bring in about $33 billion but pay out about $45 billion.

For the mass-transit account, revenue is expected to total $5 billion while outlays total $9 billion.

“The tax has not kept up with the actual costs,” Nigro said.

Vehicles with increasingly higher gas mileage and a drop in overall driving have compounded the shortfall.

To make up the deficit, the federal government since 2008 has transferred a total of $53 billion from the general treasury to the highway fund.

Democrats and President Obama have repeatedly called for more spending on roads and other infrastructure. On Monday, the Obama administration released a “Rebuild America” score card on the dismal state of the nation’s roads and bridges.

Washington’s roads, for instance, are in the seventh-worst shape in the nation, with 22 percent of 83,743 miles of public roads deemed in poor condition. Of the state’s 7,920 bridges, 26 percent are considered structurally deficient.

Bills have been introduced in the current Congress to raise the federal gas tax, with a bipartisan Senate proposal calling for a 12-cent increase and a House bill asking for a 15-cent increase. The latter, introduced by Democrat Rep. Earl Blumenauer of Oregon, has no co-sponsors.

Still, higher gas taxes are necessary for any long-term fix to sustain the highway fund, said Rep. Rick Larsen, D-Everett, member of the House Transportation and Infrastructure Committee. Larsen has criticized the “MacGyver approach” to patching up the highway fund, with financial gimmicks in place of the duct tape TV’s secret agent MacGyver might have used.

Larsen said one disagreement between Democrats and Republicans is whether the federal government should bear the cost of infrastructure upgrades. Not all states, Larsen believes, can be relied on to make the necessary investments.

Several states recently voted for higher gas taxes. But Washington’s Legislature two years in a row failed to pass a transportation funding package that included an increase in the state’s gas-tax rate of 37.5 cents a gallon.

Seattle Times news researcher Miyoko Wolf contributed to this report. Kyung Song: 202-383-6108 or ksong@seattletimes.com. Twitter: @KyungMSong