WASHINGTON — As the summer driving season swings into full gear, states can expect a large pothole in their construction budgets if Congress doesn’t reach an agreement quickly on how to pay for federal highway and transit programs, President Barack Obama and his top officials are warning.

States will begin to feel the pain of cutbacks in federal aid as soon as the first week in August if lawmakers don’t act, Transportation Secretary Anthony Foxx said in a letter to states. That’s because the balance in the federal Highway Trust Fund is dropping and will soon go below $4 billion, the cushion federal officials say is needed for incoming fuel tax revenue to cover outgoing payments to states.

The cuts will vary from state to state but will average about 28 percent, transportation officials said.

By the end of August, the trust fund’s balance is forecast to fall to zero and the cuts could deepen.

A second deadline is coming Sept. 30 when the government’s authority to spend money on transportation programs expires.

Revenue from federal gas and diesel taxes continues to flow into the trust fund, but the total is expected to be about $8 billion short of the transportation aid the government has allocated to states this year. Over the next six years, a gap of about $100 billion is forecast.

At the same time, transportation experts and industries that depend on the highways to get their products to market are calling for greater spending on transportation to shore up aging roads, bridges and tunnels.

“Right now there are more than 100,000 active projects across the country where workers are paving roads and rebuilding bridges and modernizing our transit systems,” Obama said. “And soon states may have to choose which projects to continue and which ones to put the brakes on because they’re running out of money.”

Already some states are cutting back on construction projects because of the uncertainty of federal funding, Foxx told reporters earlier. “I think people will see it in the traffic. I think people will see it in the condition of our roads,” he said.

The reason for the shortfall is that revenue from the federal 18.4-cent-a-gallon gasoline and 24.4-cent-a-gallon diesel tax hasn’t kept pace with transportation needs. The taxes haven’t been increased in more than 20 years, while construction and other costs continue to go up.

The most obvious solution is to raise fuel taxes, which is what several blue-ribbon commissions have recommended and business groups like the U.S. Chamber of Commerce and the American Trucking Associations have urged. But neither political party nor the White House wants to get out front on a proposal to raise taxes in an election year.