An intelligence community watchdog gave preferential treatment to House Democrats conducting an impeachment inquiry into President Trump while rebuffing Senate Republican questions, according to Republican Arkansas Sen. Tom Cotton.

“I’m dissatisfied, to put it mildly, with your refusal to answer my questions, while more fully briefing the three-ring circus that the House Intelligence Committee has become,” Cotton, who sits on the Senate panel overseeing the intelligence community, wrote to Inspector General Michael Atkinson.

Cotton wanted Atkinson to reveal the information that might suggest political bias on the part of the whistleblower who reported on Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky. Atkinson refused to describe the “indicia of an arguable political bias on the part of the complainant in favor of a rival political candidate,” to Cotton’s annoyance.

“This information is, of course, unclassified and we were meeting in a closed setting,” Cotton wrote. “Yet you moralized about how you were duty bound not to share even a hint of this political bias with us.”

The House Intelligence Committee, which is controlled by Democrats, learned more information from Atkinson. “The inspector general revealed that the whistleblower's possible bias was not that he was simply a registered Democrat,” the Washington Examiner reported Wednesday, citing three sources with knowledge of what was said in a closed-door briefing. “It was that he had a significant tie to one of the Democratic presidential candidates currently vying to challenge President Trump in next year's election.”

The whistleblower reported that Trump had pressed Zelensky to cooperate with his personal attorney, former New York City Mayor Rudy Giuliani, in an effort to find evidence implicating former Vice President Joe Biden in a Ukrainian corruption scandal. That report spurred House Speaker Nancy Pelosi to open an impeachment inquiry into whether Trump abused his power to push foreign governments to investigate a perceived front-runner for the 2020 Democratic nomination, an investigation that gained momentum after Trump called publicly for both Ukraine and China to investigate the Biden family’s foreign business dealings.

"Your disappointing testimony to the Senate Intelligence Committee on September 26 was evasive to the point of being insolent and obstructive," Cotton wrote, before demanding that Atkinson identify any Democratic candidate who has or had a professional relationship with the whistleblower. “This information is urgently relevant for the American people and their elected representatives to evaluate the complainant’s credibility and to determine whether the House’s so-called impeachment inquiry has been, in reality, a well-coordinated partisan attack from the beginning.”