"Each and every month, we are proving that you don't need dark money to make real change happen," said Montana Democratic Sen. Jon Tester in an email sent to supporters on Monday.

That's certainly easy for Tester to say. It's only July of the off year, and already the dark money group Majority Forward is spendng money to make sure he's re-elected. The group included Tester in a $3.5 million ad buy across six different states in May, touting his record to voters who will decide whether or not the Democratic senator keeps his seat come 2018. "This is an opportunity to let working families know what their senator is doing to make a difference," the group's president explained at the time.

Majority Forward, the nonprofit affiliate of Senate Majority PAC, does not disclose its donors, pouring millions of dollars from unknown sources into states across the country to support Democratic candidates. The group says its primary mission "is to encourage full participation by voters in our election process."

In the email, Tester boasted that his campaign's average online donation is $21.80. "That's a hard working Montanan in Havre or Harlowton giving what she can to make a difference, not a special interest based in Washington buying influence," Tester said.

On its website, Majority Forward lists an address in downtown Washington, nearly 2,000 miles outside of Billings.

If Tester is so opposed to dark money pouring into his state from Washington, will the senator denounce Majority Forward's efforts on his behalf?

Emily Jashinsky is a commentary writer for the Washington Examiner.