Attendees of the billionaire Koch brothers’ strategy session in Rancho Mirage, California, this weekend were met with an unusual sight: A blimp overhead declaring “Koch brothers — dirty money.”

The blimp was the work of Greenpeace, the environmentalist group that last year brought attention to the prominent role that Charles and David Koch, owners of Kansas-based oil company Koch Industries, play in efforts to discredit climate change theories.

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The brothers, who jointly own the second-largest privately held company in the US, “are able to push their polluter agenda through tens of millions of dollars in campaign contributions, lobbying, and funding fronts groups and think tanks,” Greenpeace said on its website.

The environmental group argued that Koch brothers meetings attract enough big conservative money for the meetings to be considered a de facto political movement in and of itself.

Analyzing the attendance list of an earlier Koch meeting in Aspen, Colorado, Greenpeace found that the participants had contributed more than $61 million to political campaigns since 1990 — a statistic that prompted the Guardian‘s Ed Pilkington to declare that the meetings’ attendees form “a major, though unofficial, bloc within American politics.”

Since Greenpeace’s investigation last year, the Koch brothers have come under intense scrutiny from liberal activists, who see in the energy billionaires’ efforts an attempt to manipulate the American political process.

Among the groups organizing to counteract the Kochs’ perceived influence is Common Cause, which held a discussion panel and rally at Rancho Mirage on the day the Koch brothers’ meeting began.

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A letter Charles Koch sent last fall promoting this weekend’s meeting was at the heart of a complaint Common Cause filed with the Department of Justice earlier this month. The liberal activist group asked the department to look into “conflicts of interest” that two Supreme Court justices, Antonin Scalia and Clarence Thomas, may have had when they ruled on Citizens United last year.

That ruling, which upended decades of campaign spending restrictions by corporations and unions, is seen by some to have benefited the Kochs, who have funded a number of conservative groups that were able to campaign without restriction in last year’s elections.

Common Cause argued that, by attending Koch brothers events, the justices compromised their credibility in ruling to strike down campaign spending laws.

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“It appears both justices have participated in political strategy sessions, perhaps while the case was pending, with corporate leaders whose political aims were advanced by the decision,” the group said.

While the Koch brothers have been holding twice-yearly meetings for years, the latest event in Rancho Mirage was unique for the amount of attention it received from progressive activists. The presence of progressive rallies and events suggests that the days of the Koch brothers operating with anonymity are gone.

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“The Koch brothers manage to be destructive in so many areas,” Jodie Evans of Code Pink said at the Common Cause rally, as quoted at the Guardian. “But one positive thing they’ve done is to galvanize so many different opponents around them.”

The following video was posted to YouTube by Greenpeace.