Washington (CNN) The Democratic lawmakers leading the case against President Donald Trump accused White House counsel Pat Cipollone of being a "fact witness" to Trump's wrongdoing and demanded on Tuesday that he disclose what he knows to the Chief Justice.

In a letter to Cipollone , the House managers argue that Cipollone has "detailed knowledge of the facts" regarding Trump's dealings with Ukraine, that he "played an instrumental role" implementing Trump's directive to block witnesses and testimony, and that he therefore must disclose what he knows so that the "Senate and Chief Justice can be apprised of any potential ethical issues, conflicts, or biases."

Democrats have essentially no power on their own to force Cipollone to do anything. If they believe he has critical firsthand information, they could push for a vote on whether to depose him or call him for public testimony, but that would require Republican support.

Still, the move was the latest in an escalating tit-for-tat between House Democrats and the White House. Trump has been saying for months that Democratic Rep. Adam Schiff, who is leading the House managers and oversaw the inquiry in the House Intelligence Committee, illegally coordinated with the Ukraine whistleblower and should be required to testify at the trial.

The Democrats' letter said Cipollone's situation "threatens to undermine the integrity of the pending trial," ramping up their criticism of the Republican-led process that they've labeled a "cover-up."

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