The meanest U.S. Senate races of all

“If you can’t say anything nice, don’t say anything at all.”

It's hardly the motherly mantra to which politicians adhere.

As another bitter election season concludes, the airwaves will temporarily clear of the nasty political ads that have dominated them. In the 34 states with Senate seats up for grabs, candidates and political groups ran more than 1 million TV ads to influence those races, according to a Center for Public Integrity analysis of data from Kantar Media/CMAG, an advertising tracking firm.

About 46 percent of those U.S. Senate ads were straight-up attack ads, but in some states it was far more than half. Many more ads that didn't qualify as "negative" contained a mix of positive and negative messages.

The closest races had the most mudslinging. In North Carolina, where the U.S. Sen. Kay Hagan, a Democrat, and Thom Tillis, a Republican, have been locked in a dead heat, nearly seven out of 10 ads were overtly negative, the highest rate. Many more contained a mixture of positive and negative messages.

Kansas, Iowa, Michigan and Colorado are also notoriously bitter affairs, where about 60 percent of the attack ads that aired were political ads.

The following graphic, courtesy of Center for Public Integrity partner Slate, tells the story:

Related: Percent of Senate election ads that were attack ads

Click here to read this story at the Center for Public Integrity

This story is part of Buying the Senate 2014. Whether Republicans control both chambers of Congress squarely depends on Senate races in a handful of states. Click here to read more stories in this investigation.

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Copyright 2014 The Center for Public Integrity. This story was published by The Center for Public Integrity, a nonprofit, nonpartisan investigative news organization in Washington, D.C.