So what's the answer? Fortunately, the folks at the Wesleyan Media Project can help us answer this question--at least from 2000-2010. They combined their 2010 data with previous data from the Wisconsin Advertising Project and compare the frequency of televised attack ads to promote ads to contrast ads. Contrast ads "mention both a favored candidate and his or her opponent" while attack ads "mention only the candidate being attacked." Promote ads just mention the candidate sponsoring the ad.
They conclude that "the 2010 House and Senate advertising is the most negative in the past decade." Further, although the Republicans sponsor slightly more attack ads than the Democrats, attack ads are on the rise in both parties. Below, I've graphed the Wesleyan/Wisconsin data and separated it out by party. The message here is pretty clear.
Here are the data for the Democratic sponsored ads:
And the Republicans:
Brace yourself for more negativity in 2012.