“We have truly crossed a new Rubicon,” one campaign finance expert says in response to new data showing record levels of outside spending pouring into high-stakes Senate races this election cycle.

According to an analysis released Tuesday by the Brennan Center for Justice at NYU School of Law, outside groups—free from contribution limits and sometimes able to conceal their donors from the public—are outspending both political parties and candidates “in the battle for control of the U.S. Senate.”

In fact, of the $557 million spent so far in 10 key Senate contests, the Brennan Center says such groups are responsible for $282 million, or 51 percent. Further, in each of the four most expensive contests—Nevada, New Hampshire, Ohio, and Pennsylvania—candidates account for 37 percent or less of total spending.

“This may be a new high for non-party outside spending, although direct comparisons to past cycles are hindered by a lack of available data,” the report states.

The analysis takes into account “shadow party groups—super PACs and nonprofits run by former top staff of party committees or party leaders and able to mimic party spending strategy—as nonparty outside groups,” the Brennan Center explains. “This is because these groups, despite being controlled to some degree by the parties, take unlimited contributions and sometimes hide their donors. That the parties are increasingly outsourcing their finances to unregulated shadow parties severely weakens the campaign finance system’s protections against corruption and undue influence.”

Among the report’s other notable findings:

  • Nearly 60 percent of the money spent so far in top Senate races supports GOP candidates, a reversal from 2014. Dark money spending, from groups that conceal their donors, favors GOP candidates six to one.
  • Just three groups, One Nation, with ties to Republican Majority Leader Mitch McConnell; the Koch network’s Americans for Prosperity; and the U.S. Chamber of Commerce, account for 67 percent of all dark money spending.