(CNN) A mystery super PAC that spent nearly $2 million meddling in the Arizona Republican Senate primary was revealed to be funded by Democrats after the group's August Federal Election Commission report was published last weekend.

That super PAC, blandly titled "Red and Gold," was able to conceal its donors -- and thus its links to national Democrats -- and avoid disclosure until after the election with a tactic increasingly used by other Democratic super PACs this cycle.

Last May , a pair of Democratic super PACs meddled anonymously in the West Virginia Republican Senate primary using the same arrangement.

In Arizona, like in West Virginia, the super PAC avoided disclosure by forming on August 1, within a month of the August 30 primary, and electing to file monthly FEC reports. Per FEC requirements, that meant the group wouldn't file a report detailing its fundraising and spending until September 20, nearly a month after the contest took place.

By forming shortly before the contest, getting a rush of contributions from other PACs and donors, and bombarding voters with ads over the final month, the Democratic groups played a significant, and anonymous, role in the election even over a short period of time.

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