Carl Bernstein, one of the investigative reporters who exposed the Watergate scandal, has accused Republican senators of a "cover-up, plain and simple," in President Donald Trump's impeachment trial.

Bernstein was referring specifically to the fading prospects that new witnesses would be called after Republican Sen. Lamar Alexander announced on Thursday that he would not vote in favor of the motion.

The motion to call witnesses requires a simple majority in the Republican-controlled Senate, requiring at least four GOP senators to side with the Democrats. The Senate votes Friday evening.

"We have seen now a really shameful episode in our history that is going to read down for many, many years," Bernstein told CNN's Anderson Cooper.

Bernstein has previously drawn parallels between the Watergate scandal, which resulted in President Richard Nixon's resignation, and Trump's impeachment.

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Carl Bernstein, one of the two investigative reporters who exposed the 1970s Watergate scandal, has accused Senate Republicans of abetting a cover-up as the prospects of calling new witnesses in President Donald Trump's impeachment trial continue to fade.

"They have covered up what the president of the United States has done in his grievous action when they had the ability to find out more and reach a bipartisan, as it were, decision if we could hear from the witnesses," Bernstein told CNN's Anderson Cooper on Thursday night.

He then described the action as a "cover-up, plain and simple."

'The so-called world's greatest deliberative body'

"And there has been no attempt throughout this proceeding by the Republicans in this Senate of the United States, the so-called world's greatest deliberative body," he said.

"We now can see how deliberative it is, that we have seen now a really shameful episode in our history that is going to read down for many, many years."

Bernstein's remarks came after Republican Sen. Lamar Alexander effectively ended Democrats' push for witnesses to be heard in Trump's impeachment trial by announcing he was opposed to the move.

New reporting Sunday on details from the coming memoir of Trump's former national security adviser John Bolton had given impetus to the push.

But with Alexander's vote considered one of the four that Democrats had a chance of obtaining to hear from Bolton, and only two having indicated they are in favor, Alexander's statement means the trial is likely to conclude Friday.

Sen. Lamar Alexander. AP

Bernstein and his reporting partner at The Washington Post, Bob Woodward, played a central role in exposing President Richard Nixon's cover-up of a break-in at the Democratic National Committee offices at the Watergate complex in 1972.

It ultimately led Nixon to resign before he could be impeached by Congress.

Bernstein has long been an adamant critic of Trump, and he has compared the campaign to drum up dirt on a domestic rival — which got Trump impeached — to the "dirty tricks" that resulted in Nixon's impeachment investigation.

"Watergate was an attempt by Richard Nixon to undermine the democratic electoral process in the United States, our basis of democracy, the electoral system, by trying through political espionage and sabotage," he told CNN last September.

In 2018, Bernstein also argued the Trump presidency was "worse than Watergate" and said Republicans on Capitol Hill had "done almost everything they can to impede and undermine legitimate investigation" of Trump.

Woodward, his reporting partner, also published an explosive book — titled "Fear" — detailing multiple damning allegations about the Trump administration, and portraying his White House as chaotic and disloyal.