Sen. Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE (D-Conn.) is requesting an investigation into a group of executive branch agencies that are cooperating or considering cooperating with congressional probes into Hunter Biden and his work related to the Ukrainian gas company Burisma.

In a letter sent to the inspectors general at the National Archives, State Department, Treasury Department and Department of Homeland Security, Murphy expressed concern that the agencies are being "weaponized" by President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE to hurt his political opponents. He argued that the same agencies refused to cooperate with "legitimate congressional investigations" into Trump, including ones focused on bringing articles of impeachment against the president.

"I am deeply concerned that the National Archives and the Departments of State, Treasury and Homeland Security may not be applying a consistent test regarding inquiries made by Congress, choosing to comply with the requests that are designed to damage the president’s political opponents, but refusing to comply with requests into the activities of the president," Murphy wrote. "This double standard threatens to make these agencies agents of the president’s political campaign, in contravention of federal law."

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Politico first reported on the letter.

Murphy called on the inspectors general at the various agencies to answer a list of questions on the matter by March 23. Among other questions, the senator asked the agencies to provide information on the extent of its cooperation with requests related to Biden, the son of former Vice President Joe Biden Joe BidenCast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response Biden tells CNN town hall that he has benefited from white privilege MORE, a leading Democratic presidential. He also asked whether a different standard has been applied to congressional investigations into the president and those into his political opponents.

"It is critical that executive branch agencies remain apolitical and not be made agents of a president’s political operation or reelection campaign," he said.

Trump's request for the Ukrainian president to investigate the Bidens over unfounded allegations of corruption during a phone call in July became central to the president's impeachment. The Senate acquitted Trump last month.

The corruption allegations stem from Hunter Biden's time on the board of Burisma Holdings while his father led the Obama administration's anti-corruption efforts in Ukraine.

Sen. Ron Johnson Ronald (Ron) Harold JohnsonSecond GOP senator to quarantine after exposure to coronavirus GOP-led panel to hear from former official who said Burisma was not a factor in US policy The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump contradicts CDC director on vaccine, masks MORE (R-Wis.), chairman of the Senate Homeland Security and Governmental Affairs Committee, and Sen. Chuck Grassley Charles (Chuck) Ernest GrassleySenate Republicans signal openness to working with Biden Senators offer disaster tax relief bill Trump spikes political football with return of Big Ten season MORE (R-Iowa) are now conducting a broad probe that partly touches on Hunter Biden's stint on the board of Burisma. Johnson indicated on Monday that he would move forward with a subpoena vote related to the investigation.

The two have also reportedly made requests to the State Department and National Archives and Records Administration for documents related to Hunter Biden and other officials.