As of Monday, pro-Republican third-party organizations had paid for a total of $23.6 million worth of ads, while Democratic-aligned groups had spent just $4.8 million on TV.

For the next month — the crucial period during which many voters begin to consider their choices and make up their minds — the disparity is even more daunting for Democrats: Between now and Oct. 20, groups backing Republicans have $9.4 million worth of TV ads reserved across 40 districts, while outfits supporting Democrats have put down only $1.3 million in five districts.

“It’s frustrating down here because we know if we get our message out, people will agree with us, but when airwaves are flooded like they are, democracy and public debate lose,” said Rep. Tom Perriello (D-Va.), a heavily targeted freshman, in a phone interview in between campaign events in his Southside Virginia district.