One of special counsel Robert Mueller's top prosecutors is offering advice to House Democrats for their impeachment proceedings against President Trump.

Andrew Weissmann, a former Justice Department official who was known as Mueller's "pitbull" during the Russia investigation, made his debut Wednesday as an NBC News legal analyst.

He explained how the House Democrats faltered in their first day of public hearings and should refocus their proceedings from Ukraine to United States election interference if they want to make an effective case against Trump.

"The key thing that the Democrats have to think about is, where are you going to be at the end? What is it that you’re going to be asking people to really care about? And you need to find out — and make the case for — why should there be impeachment where people vote to convict as opposed to acquit now, and not sort of let it go to the election?" Weissmann said on MSNBC.

He said Democrats "needed to focus on that this was about election interference in our election" and encouraged more analogies to the Watergate scandal that led to President Richard Nixon's resignation.

During an earlier appearance on NBC News, Weissmann also critiqued the Republicans' performance during the House Intelligence Committee hearing on Wednesday. Claiming the facts were not on their side, Weissmann said GOP lawmakers "raised a lot of confusion" as a defense counsel might do in a criminal case under similar circumstances.

Impeachment proceedings were touched off by a whistleblower complaint about a July 25 phone call in which Trump urged Ukrainian President Volodymyr Zelensky to look into a conspiracy theory regarding CrowdStrike. He also urged him to investigate former Vice President Joe Biden and his son Hunter Biden's business dealings in Ukraine. The elder Biden is a leading candidate for the 2020 Democratic presidential nomination.

One of the witnesses who testified on Wednesday, acting U.S. Ambassador to Ukraine William Taylor, said he was told military aid to Ukraine was contingent on Ukraine publicly announcing these investigations. The congressionally mandated security assistance, meant to counter Russian aggression, was eventually given to Ukraine, but only after lawmakers complained and the whistleblower complaint was submitted to the Intelligence Community inspector general. The president claims he urged Ukraine to open the investigations to root out corruption in Ukraine.

"It's telling that he said it's really important that it be public," Weissmann said. "I don't really care whether you do the investigation. What's important is it be a public announcement of the investigation," he added.

Trump told reporters on Wednesday he does not recall a July 26 phone call with European Union Ambassador Gordon Sondland, another impeachment witness, in which he was said to have asked about "the investigations."

Weissmann said this could be a "huge mistake" for Trump because Sondland and an aide to Taylor, David Holmes, who allegedly heard Trump on the call, could claim under oath that he said this, and the president "can't rebut it" unless he suddenly says he recalls the conversation.