In a report this summer, the Brookings Institute looked at the advertising for the primary season and found that both parties exceeded expectations when it came to negative campaign ads. Likely due to the extra-long campaign season, Democratic ads nearly matched the worst primary in 36 years, Bill Clinton's 1992 win. No one likes inter-party fighting, but when Dukakis and Gore faced little negative ads in the primary, they lost in the general election.
On the Republican side, campaigns seem to be full of bile over the past several cycles. But 2008 wasn't as nasty, possibly because of the struggle for funding suffered by several candidates.
The institute also analyzed commercials for content on specific policies, with both parties contributing about 28 percent of their air time to what really matters — the issues.