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Editor’s note: The Center for Public Integrity is tracking political advertising in races for the U.S. Senate and state-level offices. Use these two, interactive features — with new data every Thursday — to see who is calling the shots and where the money is being spent.

Politicians’ battle for the U.S. Senate has taken a turn toward the dark side.

More than seven in 10 U.S. Senate-focused television advertisements last week attacked or criticized a political candidate either in full or in part, according to the Center for Public Integrity’s analysis of preliminary data from Kantar Media/CMAG, an ad tracking firm.

Only about 28 percent of U.S. Senate-focused television advertisements last week contained a “positive” message meant to promote, not attack, a candidate, the data indicates.

That’s even more negativity than the week before, which, percentage-wise, featured marginally more positive-sounding ads. Campaigns seemed downright cheery two months ago, when about 42 percent of U.S. Senate-directed ads featured a positive message during that month’s second week.

Crown the Democratic Senatorial Campaign Committee as last week’s ultimate hater.

Not only did it produce more TV ads than any other candidate, political committee or nonprofit group, but all 6,800 or so of them — across 10 battleground states — contained content that overwhelmingly slammed a Republican U.S. Senate candidate rather than promote a Democrat.

About 1,500 negative DSCC ads hit Iowa alone, pummeling Republican Joni Ernst, who’s running in a close race against Democrat Bruce Braley, a sitting congressman.

The candidates largely stayed above the fray and let others play dirty.

The more than 1,500 ads that Ernst’s own campaign ran last week were all positive, while most of the nearly 1,000 ads Braley’s campaign sponsored were similarly sunny. But other groups on both sides of the partisan divide — including conservative super PAC American Crossroads and the liberal Sierra Club — slung mud at either Ernst or Braley.

Nonprofit organization Crossroads GPS ranks tops among conservatives’ negative Nancys with more than 3,500 U.S. Senate-focused attack ads splashed across Arkansas, Colorado, Kentucky, Louisiana and North Carolina. The National Republican Senatorial Committee produced about 1,900 demonstrably nasty ads targeting Alaska, Arkansas, Colorado, Iowa or North Carolina.

But Senate election ugliness smacked Kansas harder than any other state last week, where just 4 percent of TV ads in this tossup race contained a “positive” message that promoted either Republican incumbent Pat Roberts or his independent challenger, Greg Orman.

The rest of the ads either contrasted the candidates in stark terms or outright bashed one of the candidates. Overall, about 3,100 TV ads aired last week in Kansas —an 80 percent increase when compared to the week before, according to the analysis of Kantar Media/CMAG data.

Related: Big money is also being spent in elections for governors, including millions from two national groups funded by billionaires, unions and energy companies.

Neither the campaigns of Roberts or Orman produced a single TV ad with a “positive” tone during the past two weeks. Last week alone, Roberts’ campaign aired about 1,000 TV ads that attacked Orman. Orman’s campaign, for its part, produced about 900 that at least in part criticized Roberts.

Several other key U.S. Senate races were similarly uncivil.

Among the snippiest last week: New Hampshire, where Democratic Sen. Jeanne Shaheen faces Republican Scott Brown (8 percent of TV ads were positive). That wasn’t a downer for WMUR-TV 9, New Hampshire’s only network affiliated station, which aired 830 Senate-focused ads last week — more than any other station nationwide. It also led all stations the week before.

North Carolina’s Senate race, where Democratic Sen. Kay Hagan faces Republican Thom Tillis, wasn’t much more positive than the New Hampshire race: 10 percent of ads there were positive.

Such a scorched-earth strategy isn’t without risk.

“When both sides engage in high levels of attack with little advocacy, voters may decide — a pox on both your houses — to simply stay home,” said Kathleen Hall Jamieson, director of the Annenberg Public Policy Center at the University of Pennsylvania who’s work focuses heavily on political communication. “Importantly, voters who know only why to vote against candidates will have trouble seeing the connection between campaigning and governance.”

The most upbeat Senate race last week? The suddenly whitehot South Dakota contest, which attracted more than 1,700 Senate-focused ads last week — the vast majority positive —now that national Democrats and campaign finance reform outfit Mayday PAC have deemed the race in play. Republican Mike Rounds, Democrat Rick Weiland, Democrat and independent Larry Pressler are all polling close to one another.

In all, about 66,000 TV ads targeted U.S. Senate races last week. That’s up from about 57,000 the week before.

Republicans must capture six seats to seize control of the U.S. Senate from Democrats.

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