FILE - In this Dec. 18, 2014 file photo, gas is pumped into a car at the Eastcoast filling station in Pennsauken N.J. (Matt Rourke/AP)

THE PRICE of crude oil dipped below $50 a barrel on Monday. National gasoline prices average a little over $2 a gallon. On principle and on politics, now is the best time Washington has seen in years to raise the federal gas tax.

Congress last year refilled the Highway Trust Fund, which pays for a range of transportation projects across the country, with a familiar strategy: short-term budget gimmickry. Instead of providing local transportation planners with the certainty they need to plan and execute large projects, Congress set the trust fund to run dry again in May. Instead of agreeing on a robust infrastructure policy that invests rationally in the economy, lawmakers scraped another $11 billion from corners of the federal budget.

The reason is simple: Members of Congress don’t want to raise the gas tax, unchanged at 18.4 cents per gallon since 1993. The gas tax operates on a straightforward principle: Those who use the roads should pay for them. But over the past two decades, the value of the revenue the tax produces has dropped by about a third — partly because of inflation and partly because cars have become more fuel-efficient. This constrains the amount of money Congress is willing to put into transportation and encourages overuse of the roads at the expense of the general taxpayer.

Lawmakers could have headed off some of the problem in 1993 by indexing the gas tax to inflation, but they put that responsibility on later congresses, which failed to act responsibly. Now, with lower oil prices, the politics of raising the gas tax should be easier, the potential backlash blunted by Americans who don’t feel as pinched at the pump. True, lower gas prices will stimulate the economy to some degree, and raising the gas tax would reduce that effect. But raising it now would restrain present and future demand for gasoline, encouraging Americans to maintain some of the resilience to oil price volatility that the country built up in recent years — and possibly even checking future price spikes.

Many in Congress understand the case for raising the gas tax, and some are willing to say so. Sen. Chris Murphy (D-Conn.) and Sen. Bob Corker (R-Tenn.) proposed last year to raise the gas tax by 12 cents per gallon over two years, bringing the tax to about the level it would be now if it had grown with inflation. Their plan would also peg the level to future inflation.

The bill didn’t move. But Sen. John Thune (R-S.D.), the new Commerce, Science and Transportation Committee chairman, did not rule out raising the gas tax when asked about it on Sunday. “We have to look at all the options,” he told “Fox News Sunday.” “I don’t think we take anything off the table at this point.”

That’s no ringing endorsement. But it leaves some room for doing the right thing.