Montana's Democratic Gov. Steve Bullock waited until just before the filing deadline in March to announce he would take on incumbent Republican Steve Daines in the race for Senate.
It took a little more than a week for millions of dollars in campaign contributions to begin pouring into the sparsely populated state from outside, quickly flooding Montana's small broadcast markets with campaign ads, many of them decidedly negative.
"Whether it's the radio or you're watching the nightly news or Jeopardy!, it's political ad after political ad after political ad," says University of Montana journalism professor Lee Banville.
With control of the Senate at stake in next week's election, a tidal wave of outside money is washing over states with the most competitive races, such as Arizona, Iowa and Montana.
Much of that money is flooding in from groups that support but are not affiliated with the candidates. Unlike the candidates, however, these outside groups are not required to reveal their donors, leaving voters in the dark about who's helping pay for the ads.
"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," says Michael Beckel, research director at Issue One, a campaign finance reform group.
The numbers are staggering: Nearly a billion dollars in outside money has flooded into Senate races with much of it going to a handful of states where the races are especially tight, according to the Campaign Finance Institute.
Montana, for one, is the nation's sixth smallest state by population. Its largest city, Billings, has fewer than 110,000 people, enough to fill a large college football stadium.
As of this week, more than $95 million in outside money had poured into the Montana Senate race, easily dwarfing what the candidates raised themselves. Much of the money goes to pay for Internet and TV ads supporting the candidates or attacking their opponents.
That pales in comparison to the $140 million that's gone into Iowa, where Democrat Theresa Greenfield is hoping to unseat incumbent Republican Sen. Joni Ernst.
But even Iowa can't compare to the race in North Carolina between Democrat Cal Cunningham and GOP incumbent Sen. Thom Tillis. The heated race has raked in a record-breaking $182 million in money from outside groups, making it the most expensive Senate race ever, in large part through ads on TV attacking Cunningham as well as Tillis.
These groups aren't required to reveal their donors, leaving voters to guess who's behind a lot of the ads, says Chisun Lee, deputy director of the Election Reform Program at the Brennan Center.
These outside groups are also behind some of the most partisan and vitriolic ads, she says.
"If there's transparent spending and voters know exactly who is behind a message, it's less likely the message is going to be misleading or insulting or especially negative," Lee says.
As a result, money to outside groups brings a level of discord to politics that can be misleading, she adds.
"This is a very hyperpartisan environment and a very nationalized environment, so you've got out-of-state interests coming into a lot of House districts, a lot of Senate districts, because control of Congress is at stake," Beckel says.
Donors and campaigns are typically well aware who's funding these groups, Lee says.
"These are not secrets among the insiders. It's voters who are deprived of the information they need to know to evaluate the ads that they're inundated with every election season," she says.
"It's very much a bipartisan problem, and the ones that lose out are voters," Lee says.
A bill passed by the House would, among other things, require groups to reveal their large-dollar donations quickly. The proposed measure, however, has stalled in the Senate.