Outside groups supporting Republican candidates in House and Senate races across the country have been swamping their Democratic-leaning counterparts on television since early August as the midterm election season has begun heating up.

Driving the disparity in the ad wars has been an array of Republican-oriented organizations that are set up so they can accept donations of unlimited size from individuals and corporations without having to disclose them. The situation raises the possibility that a relatively small cadre of deep-pocketed donors, unknown to the general public, is shaping the battle for Congress in the early going.

The yawning gap in independent interest group spending is alarming some Democratic officials, who argue that it amounts to an effort on the part of wealthy Republican donors, as well as corporate interests, newly emboldened by regulatory changes, to buy the election.

“While each of our campaigns has the resources they need to be competitive, we now face shadow groups putting their thumbs on the scale with undisclosed, unlimited and unregulated donations,” said Senator Robert Menendez of New Jersey, chairman of the Democratic Senatorial Campaign Committee.