By Tim Hains - October 16, 2014

Unprecedented levels of outside spending have made Iowa’s Senate contest one of the most talked about of the year. How much money is being spent has itself become an issue in the race, with more than $35 million in spending by outside sources expected before Election Day.

At their second debate last Saturday, Republican hopeful Joni Ernst and her Democratic rival, Rep. Bruce Braley, sparred over the role outside money is playing in the campaign.

Ernst slammed Bruce Braley for supporting "EPA over-reaches" and receiving the backing of "extreme environmentalist from California" Tom Steyer. Braley hit back that Ernst is in the pocket of the "Koch brothers and big oil backers" who want to "get rid of" rules protecting the environment.

So, who are these billionaires and what are they saying about the candidates?

Backing the Democrat: Tom Steyer, NextGen Climate Action

Billionaire hedge fund operator-turned-environmental-crusader Steyer has pledged to spend up to $100 million in the 2014 election cycle supporting pro-environment candidates and calling out Republicans who deny climate change.

NextGen Climate Action’s latest ad in Iowa has a very familiar strategy: reach out to farmers.

"Joni Ernst denies the science of climate change, which makes drought and flooding more severe,” the ad says, citing risks for the agriculture industry. "Who's looking out for Iowa farmers?"

"Not Joni Ernst," it answers.

Backing the Republican: Charles & David Koch, Americans for Prosperity

Not surprisingly, the "big oil-backed" Koch Brothers have a different side of the story. Their group, Americans for Prosperity spent more than $130 million in the 2012 elections and is expected to spend just as much this year.

In this ad, AFP targets Braley for changing his stance on the Keystone XL pipeline and for implying that "a farmer from Iowa who never went to law school” was unqualified to chair the Senate Judiciary Committee.

AFP's latest ad, released Wednesday, gets right to the point: "We have to put up with Obama for two more years, but we can fire Braley now."

President Obama’s approval rating in Iowa is particularly low.

Braley responded by saying that a vote for Joni Ernst is a vote for the Koch Brothers. "On issue after issue [Ernst] has stood with the Koch brothers and their extreme agenda that is wrong for Iowa — from repealing the federal minimum wage to privatizing Social Security to opposing the farm bill and Renewable Fuel Stand," he said in the debate Saturday. After Election Day, he added, Ernst is "going to owe the Koch brothers everything."

(Libertarian Doug Butzier will also remain on the ballot in Iowa despite his death in a small plane crash this week. For more about Butzier, see this tribute from Jonathan Bernstein of Bloomberg: RIP, Doug Butzier, Hero of the Republic)