ARI SHAPIRO, HOST:
A record amount of outside money is flooding into Senate races across the country. Of course, control of the chamber is up for grabs in next week's election. As NPR's Jim Zarroli reports, much of the outside money comes from groups whose donors are a mystery.
JIM ZARROLI, BYLINE: Last March, a day before the filing deadline, Montana Governor Steve Bullock announced he would challenge the Republican Senate incumbent Steve Daines. And so began the epic battle of the Steves on the airwaves.
LEE BANVILLE: I think that Daines had his first attack ad up in - I would say it was less than a week.
ZARROLI: Lee Banville teaches journalism at the University of Montana. He says many millions of dollars are being spent on the race. The population of its largest city, Billings, can fit inside a big college football stadium, so all those millions buy a lot of TV and Internet ads.
BANVILLE: Whether it's the radio or, you know, gosh knows, if you're watching the nightly news or "Jeopardy," you just - it's political ad after political ad after political ad.
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UNIDENTIFIED PERSON #1: We've seen the climate changing right here in Montana. That's why we need to plan our votes for climate champions like Steve Bullock.
UNIDENTIFIED PERSON #2: Montana's hard-working families are keeping more of what they earn, and our small businesses are growing and hiring locally because Steve Daines fought to cut their taxes.
ZARROLI: Of the money being spent, nearly $100 million has come from outside Montana. It easily dwarfs the money the candidates have raised themselves. The same thing is happening in Iowa, Michigan, Arizona and other states with competitive races, says Michael Beckel of the campaign finance reform group Issue One.
MICHAEL BECKEL: You're really seeing both Democrats and Republicans trying to use every weapon in their arsenal to control as much power as possible in the next Congress.
ZARROLI: Some of these groups are well-known, but who is giving them money can be a big mystery to voters. Federal law allows nonprofit groups to shovel unlimited cash into campaign ads, and they don't have to reveal their donors, says Chisun Lee of the Brennan Center.
CHISUN LEE: We've seen a tremendous amount of outside spending this year and a lot of it dark money spending, meaning spending where voters can't know where the money's originally coming from.
ZARROLI: Often, these outside groups are behind the most vitriolic ads, such as this one from the National Association of Gun Rights against North Carolina Senate candidate Cal Cunningham.
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UNIDENTIFIED PERSON #3: He wants to punish honest citizens with radical gun control.
ZARROLI: The election in North Carolina between Cunningham and incumbent Republican Thom Tillis is now the most expensive Senate race in history. Nearly $180 million in outside money has poured in. The airwaves are filled with slash-and-burn ads.
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UNIDENTIFIED PERSON #4: What do you call a man who doesn't just accept Trump's corruption but votes for it? What do you call a man like this? You call him Thom Tillis.
ZARROLI: That ad is paid for by the "Never Trumper" Lincoln Project, one of the most prolific spenders this year. Montana writer Dan Brooks wonders if these sledgehammer-subtle negative ads turn off voters.
DAN BROOKS: From the perspective of people that I talk to about it, it just makes everyone generally angry at politics. I think that, like, just seeing it everywhere is alienating.
ZARROLI: Many of these groups have bland names that obscure who they really are. A bill passed by the House would force them to be more transparent about their funders. It's stuck on Senate Majority Leader Mitch McConnell's desk. The idea behind the bill is that voters should know where this outside money is coming from, even if they can't get rid of it altogether.
Jim Zarroli, NPR News.
(SOUNDBITE OF GOLDFRAPP SONG, "BLACK CHERRY") Transcript provided by NPR, Copyright NPR.