Washington (CNN) A leading Senate Democrat is calling for an independent investigation into whether the Trump administration improperly classified key documents that were sent to Congress for political reasons, including the transcript of President Donald Trump's July 25 call with his Ukrainian counterpart and the War Powers justification for the killing of Iranian Commander Qasem Soleimani.

In a letter sent Thursday to Gene Dodaro, the Comptroller General of the United States, Sen. Chris Murphy, specifically requested that the Government Accountability Office conduct a comparative review of documents to ensure classification levels are consistent with the nature of material held in the executive branch, according to a copy of the correspondence obtained by CNN.

The GAO is an independent federal watchdog group. It determined last month that the Trump administration had broken the law when it withheld the security aid to Ukraine last year, which had been appropriated by Congress.

"My firm belief is that the Trump administration has regularly abused classification rules to hide information from the public. This is an unacceptable and unconstitutional political tactic," Murphy said in a statement to CNN.

"I've asked the GAO to conduct a review of the classified material in the Senate to make sure it's not overly or unnecessarily classified. We've seen the administration classify information just for political purposes recently and the American people deserve better. I look forward to GAO's findings," he added.

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