House Republican Conference Chair Liz Cheney, R-Wyo., blasted the House Democrats' impeachment of President Trump on Wednesday, arguing that their case will be on record as the “thinnest in history.”

“What they’ve done here is completely abdicate their duty to the Constitution, and the voters are absolutely going to hold them to account for this,” Cheney told “Fox & Friends.”

President Trump, in a blistering, no-holds-barred six-page letter Tuesday to House Speaker Nancy Pelosi, lambasted the Democrats' impeachment inquiry as an "open war on American Democracy," writing that she has violated her oath of office and "cheapened the importance of the very ugly word, impeachment!"

TRUMP TELLS PELOSI IN BLISTERING LETTER THAT DEMS ARE 'DECLARING OPEN WAR ON AMERICAN DEMOCRACY'

The president argued that Democrats were trying to distract Americans from the strong economy and historically low unemployment numbers, and pointed out that Democrats have openly called for impeachment since the day he took office.

Cheney said that the impeachment inquiry was conducted in "secret," objecting to Republicans not being allowed to question the witnesses until public hearings began. The lawmaker also argued that Democrats have failed to prove their case.

"They’re now in a situation where they are bringing articles of impeachment to the floor and saying we should impeach the president based on the assumption of some impeachable behavior," she said.

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Trump's letter came a day before House Democrats are expected to vote along party lines to impeach him, sending the matter to the GOP-controlled Senate for a virtually certain acquittal. Moderate Democrats in districts Trump won in 2016 largely have fallen in line in favor of impeachment, with only two exceptions.

"The Constitution absolutely does not say that if you don’t like the president -- if you disagree with his policy -- that you can impeach him because you are assuming that he’s done something wrong," Cheney said.

"It’s really a sad day, a tragic day."

Fox News’ Gregg Re contributed to this report.