Staff for former special counsel Robert Mueller issued a warning to Democrats eager to have him testify about his investigation into Russian election interference.

Days after Mueller delivered shaky testimony to two congressional panels, a report said Mueller's team denied rumors that the 74-year-old's cognitive acuity was wavering and that prompted Democrats to compel him to appear publicly.

However, before and after a reluctant Mueller was subpoenaed, Mueller's staff told the Democrats, some of whom have cited Mueller's report as a blueprint for impeachment, that their boss would not stray from what was already disclosed in his unredacted 448-page report.

“That was made very, very clear,” one source familiar with the investigation told the Washington Post. “And then, once Mueller was subpoenaed, the committees were told you’re not going to get any more from him. Period.”

But in the run-up to Wednesday's hearings, House Judiciary Chairman Jerry Nadler said Democrats hoped to get Mueller to divulge information that does not appear in his report. "Hopefully we go a little further," the New York Democrat said on MSNBC last week after acknowledging their main strategy will focus on getting Mueller to recite the findings of his report in front of a live audience.

That did not come to pass. Mueller may have provided some sound bites for Democrats to capitalize on, including denying that his report "totally" exonerates President Trump, but as he stressed in his late-May public statement after the conclusion of his investigation, the former special counsel largely stuck to the script of his report.

Over two hearings before the House Judiciary and Intelligence committees on Wednesday, Mueller looked shaky during parts of the hearing and failed to recall key facts from his report.

Democrats now find themselves at an impasse on whether to engage in an impeachment inquiry against the president. Some of them are even privately questioning whether Mueller was all there mentally.

“It was a painful reminder that age catches up to all of us,” one unnamed House Democrat who questioned Mueller said. “Here you have this Vietnam hero and this post-Sept. 11 FBI director. You could tell he was having a hard time hearing and it was like, ‘Ugh! This is not how I want him to be remembered.’ ”

Although the number of Democrats backing impeachment proceedings against Trump has grown to roughly 100, a poll out Friday shows Mueller’s testimony on the Russia investigation did little to move the needle on support for such action.