Big piles of hidden cash are a cancer in the Oregon governor’s race.

A group called Priority Oregon is running ads attacking Gov. Kate Brown, a Democrat. A group called Defend Oregon’s Values has run ads attacking the Republican candidate, Rep. Knute Buehler of Bend.

Who is paying for the ads? Shouldn’t Oregonians be able to find out easily?

Priority Oregon is the bigger mystery. It was incorporated by a couple of GOP strategists. It’s what is called a 501 (c) 4, a social welfare organization under the tax code. There’s a similar prominent group that generally supports union and Democratic causes called Our Oregon, also a 501(c)(4).

Having a 501(c)(4) designation means that a nonprofit can run ads and do other political advocacy without being required to disclose where it gets its money. That is not right. It doesn’t matter what side of the political spectrum it is on or what the group is supporting. Reporting by The Oregonian and Willamette Week has revealed ties between Priority Oregon and some businesses, but finding out where the money is coming from shouldn’t require an investigation.

Defend Oregon’s Values is more open. It has disclosed that it was set up set up by Service Employees Union, Local 49, to challenge Buehler’s record. It also has said it will disclose its donors. And its donors are indeed listed in state campaign finance records. But according to those records, most of its money — more than $600,000 — has come from Kate Brown’s campaign. It’s really like an arm of Brown’s campaign.

All this is legal. Supporters of both candidates are using these tools to make it hard for Oregonians to know who is attacking Brown and Buehler. That’s a problem that needs to be corrected. If Brown or Buehler is elected, how will the public know if they are paying back secret supporters with legislative favors?