Democratic Rep. Maxine Waters Maxine Moore WatersWorse than nothing's been done since the massive Equifax hack Juan Williams: Racial shifts spark fury in Trump and his base Maxine Waters apologizes to millennials struggling to get a job MORE (Calif.) tore into President Trump Donald John TrumpREAD: Cohen testimony alleges Trump knew Stone talked with WikiLeaks about DNC emails Trump urges North Korea to denuclearize ahead of summit Venezuela's Maduro says he fears 'bad' people around Trump MORE and Republicans in Congress on Wednesday for questioning the integrity of the special counsel's investigation.

Speaking on "All In with Chris Hayes," Waters stressed that Republicans' increased attacks on special counsel Robert Mueller's team revealed the administration's desperation to stop the probe.

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"We don't know everything that's in this plot, except that they're trying to stop this investigation," Waters told Hayes. "This president does not want it to go forward and we cannot help but ask: Why?"

"What is he afraid of?" she continued. "If he has not been involved with collusion, if he has not been involved in obstruction of justice, if he has not been involved in money laundering, if he doesn't have a plot to lift the sanctions working with the Kremlin, and the oligarchs of Russia, [then] what is he afraid of?"

"Why is he going to such lengths to discredit the special counsel?" she asked.

Waters, a top critic of Trump and his administration, joined Rep. Ted Lieu (D-Calif.) and other Democrats in warning that Trump's attacks on the special counsel office could be a prelude to Mueller's firing.

"If Donald Trump wants to take the first footsteps of Richard Nixon, he's certainly welcome to try," Lieu warned Wednesday in a speech on the House floor. "But it will not end well for him."

The White House has repeatedly denied rumors that Trump is considering firing Mueller.

"As the White House has repeatedly and emphatically said for months, there is no consideration at the White House of terminating the special counsel,” Trump's lawyer Ty Cobb said last week.