When gasoline topped $4 a gallon, opponents of an increase in the gas tax argued that prices were already too high.

Now the average price of regular gas has dropped under $2.50 a gallon, but in the antitax environment that pervades Washington there is still scant support for increasing the gas tax to finance upkeep of the nation’s roadways and public transit systems.

The no-win dynamic is frustrating to advocates who hoped falling gas prices might reinvigorate the idea of raising the gas tax, which they view as one of the simplest, fairest and most efficient ways to pay for transportation repairs and improvements.

“If something like this is going to be done, now is the time to do it,” said Bob Corker, a Republican senator, who noted that gas prices in his home state of Tennessee fell below $2 a gallon this month. Senator Corker and Senator Chris Murphy, a Democrat from Connecticut, unveiled a proposal in June to raise the gas tax by 12 cents a gallon over the next two years and then index further increases to inflation.