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WASHINGTON — Democratic Sen. Chris Murphy is asking the oversight arms of four federal agencies to investigate reports that the agencies are cooperating with Republican probes into Joe Biden and his son Hunter, saying past refusals to turn over documents or comply with subpoenas in the impeachment investigation of President Donald Trump raise concerns that the agencies "may not be applying a consistent test regarding inquiries made by Congress."

In a letter to the inspectors general of the State Department, the National Archives, the Department of Homeland Security and the Treasury Department, Murphy, D-Conn., is asking for investigations into what he calls the "weaponizing" of the executive branch.

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The senator's request comes on the same day that the Senate Homeland Security Committee will vote on its first subpoena in its investigation into the Bidens and the Ukrainian energy company Burisma.

Murphy calls the agencies' responsiveness to Congress under the Trump administration "a clear double standard."

"I think democracy dies when the party in power starts to use government powers to destroy their political opponents," Murphy said in an interview. "This administration engaged in obstruction in impeachment, and now they're seeming to cough up whatever they are being asked of."

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The State Department has turned over about 2,800 documents to Republicans in their investigation of the Bidens and is expected to hand over more. The National Archives said it would turn over 10,000 documents to Republican investigators, and the Treasury Department has provided Republicans on relevant committees with rarely released financial documents pertaining to Hunter Biden's work with Burisma.

Sen. Ron Johnson, R-Wis., chairman of the Homeland Security Committee, has ramped up his investigation into Biden, who was vice president when his son sat on the board of Burisma, a situation that raised questions about a potential conflict of interest related to the senior Biden's efforts to push U.S. anti-corruption policy in Ukraine.

Murphy has asked for a response by March 23.