The negativity of political advertising is a constant complaint and has given rise to no end of proposals to regulate purely political speech despite the plain language of the First Amendment and obvious intention of the founders to prevent government from censoring criticism. The importance of this issue extends well beyond politics: With U.S. political advertising for all media expected to hit $3.3 billion in 2010, political ad spending constitutes a significant source of advertising revenue for all kinds of publishers. To put that number in perspective, it's just 1.4% of the $241 billion in advertising spending expected for 2010, but is nearly half as large the total spent on display advertising revenue (which makes up 1/3 of total online advertising revenue) in 2008. So as advertising revenues continue to decline and more advertising moves online, political ad spending is an increasingly important source of revenue for publishers of both traditional and new media.

But one of the more powerful arguments against such advertising is that it diminishes the effectiveness of advertising in general for all products and services--and potentially lowers revenue for publishers even more than is spent on political advertising. That's why some advertisers and even publishers could conceivably support restrictions on negative campaign ads. The problem with this argument is that it's just not true. That's the conclusion of this very interesting 1999 Stanford study by Shanto Iyengar and Markus Prior I just stumbled upon: "Political Advertising: What Effect on Commercial Advertisers?" The authors conclude:

Despite the inherent bias of all forms of advertising, people perceive product ads as generally truthful and interesting. In contrast, political ads are dismissed as dishonest, unappealing, and uninformative. When judged against political advertising, product advertising enjoys considerable public support....

They suggest two explanations for "the significant reputation gap between the two genres of advertising":

First, public distaste for political advertisements may stem from the belief that electoral choice and consumer choice are not equivalent activities. Purchasing a particular brand of soap or cereal is one thing, selecting the next president or senator quite another. The fact that voting is a "serious" task tends to undermine the legitimacy of "non-serious" forms of political communication. People may be averse to political advertising simply because it clashes with widespread norms concerning the nature of citizenship and campaigns for public office.

[Second]... the negativity of the message breeds distrust of the medium, and, more broadly, of the political process. Our results show clearly that voters were especially turned off by political ads for both candidates and initiatives when they featured negative appeals. Commercial advertisers seek to depict their clients in the most attractive terms possible, often entertaining and amusing the audience in the process. Positivity is the currency of product advertising. In seeking to depict their clients' opponents in the most unattractive terms possible, political advertisers typically anger, threaten, and repulse the audience. The overwhelming negativity of political advertising, while intended to weaken public support for the opposition candidate, also rubs off on the message itself.

But consumers are not so simple-minded as to be unable to distinguish between angry political ads and bright, happy dishwasher soap ads:

The advertising industry takes for granted that the public's dislike for political ads will spillover to product ads. In this study, however, we find a somewhat different result. Exposure to political ads drives down evaluations of political ads while leaving product ads untouched. In comparison with political ads, product ads appear even more attractive and credible. In other words, our results indicate a contrast effect. Watching political ads that they perceive as less truthful and appealing, people realize that product ads, relatively speaking, are not so bad. Rather than calling for a greater curbs on political advertisers, commercial advertisers should instead encourage the greater use of political advertising in general and negative political advertising in particular. If form holds and the next campaign features a sufficient number of nasty negative ads, many voters, instead of making the trip to the voting booth on election day, may well make the trek to the mall -or stay home to enjoy another Miller Light.