One of the Democrats' witnesses who testified in favor of President Trump's impeachment cautioned Thursday that in order to officially impeach the president, House Speaker Nancy Pelosi must send the articles of impeachment to the Republican-led Senate.

In a Bloomberg op-ed, Harvard legal scholar Noah Feldman said Pelosi, D-Calif., can delay sending the articles of impeachment to the Senate, but not for an "indefinite" period of time.

"Impeachment as contemplated by the Constitution does not consist merely of the vote by the House, but of the process of sending the articles to the Senate for trial. Both parts are necessary to make an impeachment under the Constitution: The House must actually send the articles and send managers to the Senate to prosecute the impeachment. And the Senate must actually hold a trial," he wrote, going on to say that if the House doesn't release the articles, Trump could legitimately declare that he was never actually impeached.

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"To be sure, if the House just never sends its articles of impeachment to the Senate, there can be no trial there. That’s what the 'sole power to impeach' means. But if the House never sends the articles, then Trump could say with strong justification that he was never actually impeached. And that’s probably not the message Congressional Democrats are hoping to send," Feldman concluded.

He wrote that the blaring headlines in major newspapers that read "Trump Impeached" were "media shorthand, not a technically correct legal statement."

Senate Majority Leader Mitch McConnell, R-Ky., returned to the Senate floor late Thursday to declare that the Senate and House Democrats were at an "impasse" over whether the House would transmit its articles of impeachment to trigger a constitutionally mandated trial.

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McConnell, speaking after a meeting with Senate Minority Leader Chuck Schumer, D-N.Y., said the top Democrat had insisted on "departing from the unanimous bipartisan precedent that 100 senators approved before the beginning of President [Bill] Clinton's trial" concerning logistics.

Schumer had requested a "special pre-trial guarantee of certain witnesses whom the House Democrats, themselves, did not bother to pursue as they assemble their case," McConnell said. He noted that in 1999, "all 100 senators endorsed a common-sense solution" to divide the process into two stages: one laying the groundwork for rules on matters such as opening statements, with another handling "mid-trial questions such as witnesses."

"Some House Democrats imply they are withholding the [impeachment] articles for some kind of leverage," McConnell said. "I admit, I'm not sure what leverage there is in refraining from sending us something we do not want. Alas, if they can figure that out, they can explain."

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Feldman testified earlier this month before the House Judiciary Committee, making the case along with two other law professors for why Trump had committed impeachable offenses. Constitutional scholar and George Washington University law professor Jonathan Turley, who was called by Republicans to the same hearing, argued Democrats had overreached and did not have adequate legal grounds for impeachment.

Fox News' Gregg Re contributed to this report.