Groups allied with the Republican Party are vastly outspending Democratic incumbents and their backers in battleground states, The Hill reported Tuesday.
In Ohio the target is Sen. Sherrod Brown, a first-term Democrat who faces Republican challenger Josh Mandel in November.
Brown's campaign manager Sarah Benzing recently sent an appeal to potential donors that read in part: "Special interests have outspent our campaign more than 10-to-1 in this race."
The bulk of the $5 million attack campaign against Brown has been funded by the U.S. Chamber of Commerce, Crossroads GPS and 60Plus Association, Brown's campaign said.
Brown didn't air his first campaign ad until late April and it was only a $150,000 buy in several cities.
In Missouri, incumbent Democrat Sen. Claire McCaskill has suffered $3.9 million in attack ads while outside groups have spent nearly $1.97 million against Sen. Jon Tester of Montana.
Sen. Charles Schumer, D-N.Y., said the uneven spending war between Republican-and-Democrat-allied outside groups is the biggest threat to Democrats' retaining control of the Senate.