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.