A new study by ad and marketing research firm PQ Media provides a revealing breakdown of the way ’08 political ad campaign spending is shaping up. According to Steve McClellan’s Adweek article, “Political Ad Spend to Soar“:

Political ad spending across all media is projected to reach $3.03 billion, and account for 67.2 percent of all political media spending in the 2008 election cycle…broadcast TV would command the largest share of political media expenditures in 2008 with 51.3 percent of the total…direct mail…is projected to generate more than $1 billion in spending for the first time in 2008…Internet ad spending is expected to exhibit the fastest growth during the 2008 campaign, up an estimated 84 percent compared with 2006…Other media projected to exhibit high double-digit gains are public relations, promotions and event marketing (56 percent), direct mail (53 percent) and broadcast TV (46.2 percent).

As for the kinds of campaigns that will do the most ad spending:

The presidential race is expected to command the largest share of spending in 2008 at 37 percent, or $1.67 billion, while the Senate and House races will account for 19.4 percent and 21.4 percent, respectively. Due to significantly fewer gubernatorial races (11 versus 36 in 2006), spending by gubernatorial candidates is expected to account for less than 4 percent of overall expenditures, while local races and spending on referendums will account for the remaining 18 percent.

Martin Kaste’s article “Democrats Take Civil Approach in TV Ads (For Now),” at NPR’s website notes the relatively tame tone of current political ads of leading Democratic presidential candidates broadcasted in early primary states. Kaste seems to attribute the temporary civility to the approaching Christmas season. Seems a little early to make nice, but it makes sense to tone down for a couple of weeks. Candidates probably shouldn’t risk looking too negative close to the holiday.
Don’t be surprised, however, if the civility truce starts curdling in ads beginning December 26. Why? Because candidates across the political spectrum know that attack ads are effective. Republican strategist Roger Stone explains it this way:

The problem with negative advertising is: It works. The very same voters who tell you in the polls that they don’t like it, that they hate it, will turn around and tell you the exact content of the ads. And we know, from intensive polling going on now, that these ads do move voters from over here to over here.

Negative ads can backfire, particularly if they degenerate into a vicious ad hominem attack. One frequently-cited example is the 1993 Canadian election in which the Conservative Party appeared to mock Liberal Party leader Jean Chrétien’s Bell’s Palsy partial facial paralysis, and the conservatives were damaged in the polls.
And accuracy of attack ads is critical, as Stone cautions:

…If so-called negative or comparison advertising is going to be effective, it should be footnoted, it should indisputable on the facts…And I think it is ineffective. Voters are not stupid. They can see through a late, unsupported charge.

There’s an ongoing debate about whether or not political attack ads have a depressing effect on voter turnout. One major academic study reported in the American Journal of Political Science indicated that attack ads do not depress turnout, and a more recent Journal of Politics article provides evidence that attack ads may often increase voter turnout.
There is an important distinction that needs to be made here — personal attacks vs. attacks on policies, although the lines between them are often blurry. What candidates should avoid is looking mean-spirited. But sharp, well-articulated critiques of policies, backed by credible alternatives, is what really makes a candidate look good. Ads that meet this standard win the support of swing voters who care about issues.