The 2014 midterm election marks the rise of the single-candidate dark money group, says report—a notable development when the balance of Senate power rests on a few key races.

Unknown donors and big-monied, outside groups are pouring record amounts of cash into key Senate races set to determine which political party will take control over the upper house come November’s election, according to a new report published Tuesday by the Brennan Center for Justice.

The report, Election Spending 2014: 9 Toss-Up Senate Races (pdf), found that outside spending by undisclosed “dark money” groups is on track to “shatter previous records.” According to newly-released data from the Federal Elections Commission, of the nine hotly-contested senate races this year—Alaska, Arkansas, Colorado, Georgia, Iowa, Kentucky, Louisiana, Michigan, and North Carolina—all but one is expected to beat the previous record for most outside money spent in a senate race, $52.4 million in Virginia in 2012.

The most expensive race in terms of overall spending, North Carolina at $64.8 million, is set to beat the record “several times over.”

“[We]ak campaign finance laws and Supreme Court decisions like Citizens United have made possible new means of pumping money into elections while avoiding regulation or scrutiny,” said report author Ian Vandewalker. “These tactics are gradually becoming the national norm, and give wealthy spenders more power than ever to buy influence over our political process and elected officials.”

According to the nonpartisan law and policy center at the New York University School of Law, nonparty outside spending through September 30 amounts to $158.6 million in these nine most competitive races. “Outside groups have spent at a furious rate,” the authors note, especially when compared with the $97 million spent on all 37 contests in the 2010 midterms.

While the biggest spender overall is a Democratic-aligned Super PAC, the Senate Majority PAC at $29,083,280, the report found that outside spending in favor of Republicans is much more likely to be dark money, which thus far make up 80 percent of nonparty outside expenditures.

Another election spending phenomena highlighted in the new report is the rise of dark money, single-candidate groups, which, according to the authors, is a notable development in an election year where the balance of power rests on a few key races.

According to the survey, “six of the eight highest-spending candidate-specific groups hide some or all of their donors, including the top candidate-specific spender overall.” Further, many of those single-candidate groups depend on contributions from “double-dipping donors,” who have already given up to their legal limit in direct contributions to the candidate’s campaign.

Single-candidate groups also accept sizable contributions from corporations and unions, which are completely prohibited from giving directly to candidates, the report notes, adding that some of these groups “got all their revenue from these entities.”

When handing down the Citizens United ruling, the Supreme Court “assumed that outside spending could not corrupt candidates because it comes from entities whose activity is independent of candidates’ campaigns,” the authors write. “The reality is that outside groups, some devoted to electing a single candidate, cooperate with candidates in many ways, potentially making their unlimited contributions as valuable to candidates as the direct contributions that are subject to strict caps.”

The BCJ report comes on the heels of another report put forth by the Wesleyan Media Project which found that outside groups are disproportionately funding advertisements favoring GOP candidates in both congressional and gubernatorial races.