Three advocacy organizations — across ideological lines — are telling congressional investigators to back off in a probe of EPA ties to a leading environmental group, the Natural Resources Defense Council.

Republican Rep. Darrell Issa of California and Sen. David Vitter of Louisiana are leading the investigation. They contend that NRDC lobbyists have exerted too much influence over EPA on the issues of carbon reduction and the proposed Pebble Mine at Bristol Bay, Alaska.

In September, Vitter, Issa and four other Republican lawmakers sent a letter to NRDC President Frances Beinecke, telling her to give their committees all NRDC communications with EPA on those two issues, going back to the day President Obama took office in 2009.

NRDC turned over about 450 emails last month. Investigators have also gotten documents from EPA — and that duplication is what led the three advocacy organizations to object. Washington Director Laura Murphy of the American Civil Liberties Union, Alliance for Justice President Nan Aron and Center for Competitive Politics President David Keating wrote to Issa and Vitter that the letter seems intended to intimidate NRDC "with the heavy hand of Congress."

They urged the lawmakers "to show more sensitivity to the First Amendment right to petition government" and to drop the sweeping request. They said investigators could subpoena agency records and officials and hold hearings, and should focus on the conduct of agency officials, not private citizens.

Murphy, Aron and Keating noted that their groups don't agree — or even have positions — on the carbon and mining issues. The ACLU and AFJ are considered liberal groups; CCP is seen as conservative. The letter went out Wednesday.

Vitter's office has described the probe as focusing on "the collusion between environmental activists, lawyers and lobbyists, billionaires and their supporting foundations who use large sums of money to influence environmental public policy."

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