U.S. Senate Democratic Leader Harry Reid (D-NV) discusses the candidacy of Republican presidential candidate Donald Trump while speaking at the Center for American Progress Action Fund in Washington, March 17, 2016. REUTERS/Stelios Varias

The race for the seat of retiring Senate Democratic Leader Harry Reid is attracting dark money from both sides.

By Ashley Balcerzak

Senate Minority Leader Harry Reid's looming retirement leaves quite the vacuum to fill in Nevada. The contest to replace one of the body's longest-serving Democrats is labeled a toss-up between former Nevada Attorney General Catherine Cortez Masto (D) and Republican Rep. Joe Heck. And with Heck running for a promotion, he leaves behind an open playing field for his District 3 seat.

Adding even more intrigue to the races: A whole lot of money coming from undisclosed sources.

The Sagebrush State's Senate contest has attracted $3.4 million so far from groups that don't identify their donors; that puts the race third in dark money dollars, close behind Pennsylvania and Ohio. The race to fill Heck's place in southern Nevada has seen nearly $2 million spent, the most dark money of any House race and well more than the majority of Senate races.

"Reid wants to preserve his legacy and Democrats want to pick up a majority, and without Nevada, that's going to be tough to do," said David Damore, professor of political at University of Nevada, Las Vegas.

On the Senate side, Cortez Masto's campaign has raised slightly more than Heck's: $8.7 million to $7.4 million. But adding dark money (and outside money overall) to the mix keeps the scales more balanced. Cortez Masto is facing $736,000 worth of attacks from the Kochs' Americans for Prosperity, the shadowy national security nonprofit SecureAmericaNow.org (headed by Allen Roth, a longtime aide to cosmetics heir Ronald Lauder) and the U.S. Chamber of Commerce. The Chamber, which has spent $18.7 million total so far this election and $2.7 million in Nevada's Senate race alone, spent $1.6 million of that to prop up retired Air Force officer Thomas Heck (who got 3 percent of the vote in the GOP primary) and $400,000 supporting Rep. Joe Heck (no relation to Thomas Heck).

"This is arguably the most important Senate race this year because it's the only Senate seat that the Dems have to defend," said political analyst Jon Ralston.

Super PACs in the race, including Freedom Partners Action Fund on the right and Senate Majority PAC on the left, have added another $4.8 million in spending - $2.8 million of it to against Cortez Masto and $2 million to hurt Heck. Though super PACs are required to disclose their donors, they don't always do so, as when they accept money from shadowy LLCs.

Last ditch effort in the House

Most of the spending by politically active nonprofits in the race to fill Heck's seat in southern Nevada saw almost $2 million in dark money - the total of which was nearly triple that of the next highest House race, Georgia's third district.

The activity centered around the crowded June 14 primary, where six Democrats and seven Republicans vied to be their party's candidate for District 3. Republican and Tea Party darling Danny Tarkanian and Democrat Jacky Rosen fought their way through to the general, but not without opposition from the dark money groups. They mostly sided with failed Republican contender and state Rep. Michael Roberson, who raised $829,000 mostly from individuals.

"That was a last ditch attempt to save Roberson," Ralston said. "It was the biggest last minute buy I've ever seen."

The group, which focuses on the national debt, boosted Roberson with $545,000 in positive messaging, and about $1 million against his primary opponents Tarkarian and Republican state Rep. Michele Fiore.

Last election cycle, Ending Spending poured the majority of its $4.2 million in independent expenditures into Colorado's Senate race, spending $1.8 million to back winning candidate Cory Gardner (R-Color.) and $1.5 million against the incumbent, former Democratic Sen. Mark Udall.

Roberson got an additional $250,000 bump in support from the 501(c) Main Street Advocacy, linked with former Minnesota Sen. Norm Coleman's conservative 501(c)(4), American Action Network.

"Roberson mostly carried the governor's agenda with him, which included a huge tax increase so he was vulnerable," Damore said.

To help turn the open seat blue, Harry Reid handpicked Jacky Rosen, a software developer and president of a synagogue in Henderson, Nev. The first time congressional candidate's campaign brought in $558,000, with top donors including employees from pro-choice PAC EMILY's List and unions.

But it's only August. Assuming history repeats itself to some degree, we should expect a lot more money to come. In 2012, less than a fourth of all outside spending had been doled out. Outside group spending for the Nevada Senate race is around $11.5 million, and for the congressional seat, close to $2 million.

There are also the megadonors to think about. Casino magnate Sheldon Adelson, who played in Nevada's 2012 Senate race, has so far contributed $2,700 to Joe Heck's campaign, and his wife Miriam gave another $5,200. Billionaire and CEO of Wynn Resorts, Steve Wynn, who has come out in support of Heck, has not yet contributed any funds, at least in traceable ways.

Political nonprofits investigator Robert Maguire contributed to this post.