Wave after wave of mailers, many of them bare-knuckle negative attacks on political candidates by their opponents, have been clogging the mail of Ohio residents for weeks.

Quite often, the mailers go beyond -- sometimes way beyond -- the admittedly low standards for political ads on television.

• The Ohio Republican Party sent a mailer with a photo of Iranian President Mahmoud Ahmadinejad on one side and Democratic presidential nominee Barack Obama on the other. The text charges that Obama would negotiate with "rogue dictators," including Cuba's Fidel Castro and North Korea's Kim Jong Il.

• The Sierra Club Political Committee mailed an 8-by-10-inch ad with a cartoon drawing of a "Big Oil" businessman smoking a cigar, sitting at a desk piled with stacks of money, and wearing a "Big Oil for McCain" button.

• Bring Ohio Back, a political action committee, mailed an even larger ad with what appeared to be a domestic-violence victim on the front and John McCain on the back. It says Republican presidential nominee McCain voted against the Violence Against Women Act.

• "Crime and Punishment" and a picture of Obama are on one side of an ad from the Ohio GOP. The flip side is a quote, "He acted more as a friend to criminals than to cops."

William Benoit, communication professor at the University of Missouri, has been analyzing the content of political mailers and TV ads for 10 years. He's pretty much seen it all.

But this year's crop of TV ads in particular was the most negative ever, Benoit said. Overall, he determined that 68 percent of Obama's ads were negative compared with 62 percent of those for McCain. Obama has aired far more ads than McCain.

"The only campaign in history that matches this level of negativity was in the first-ever presidential TV spot campaign, when Dwight Eisenhower had negative attacks in 69 percent of his ad statements," Benoit said.

He acknowledges that the mailers are even bolder, in part because many come from surrogates. In his years of study, Benoit found that direct-mail pieces coming directly from the candidates were more positive than their television advertising.

National and state political parties are a notch up on the negativity scale and third parties, often the so-called 527 groups, are the most negative of the bunch, Benoit said.

Both the McCain and Obama campaigns have used "guilt by association" mailers, he said. Democrats frequently link McCain with President Bush; Republicans pair Obama with William Ayers, a professor and former member of a protest group that bombed government buildings in the 1960s.

Benoit said mailers are often focused toward specific voting groups, such as working mothers or military veterans.

Scott Borgemenke, chief strategist for Ohio House Republicans, said the negative mailers aren't "a good thing for the system. Nobody feels good about this."

Benoit said a great deal of study has been done on the impact of negative versus positive mailers and ads. He said there is "no overall evidence that negative ads work better than positive ads."

Contrary to what would seem logical, mailers and ads increase in negativity when the margin in a campaign is wider and are more positive when the race is close, Benoit said.

ajohnson@dispatch.com