Faced with the pressure on Trump, the Strzok-Page texts have given Republicans a wedge to try and undermine Mueller's overall credibility.

The catalogue of some 50,000 texts show the two, who were having an affair while taking part in the FBI's 2016 investigation of Trump's Democratic election rival Hillary Clinton, repeatedly expressing political preference for Clinton and disdain for Trump.

That investigation, into Clinton's misuse of a private email server while she was secretary of state, ended when Comey decided there was not enough evidence to charge her, raising howls from Republicans of bias.

Then in June 2017, Strzok was named a top investigator in Mueller's probe of possible Trump campaign collusion with Russians. Mueller dismissed him two months later, after the affair and text messages came to his attention.

But Republicans have continued to seize on the text messages as evidence of bias in the Justice Department and FBI.

"We know that Strzok and Page had an intense anti-Trump bias," Congressman John Ratcliffe told Fox News Monday, citing indications that in the "aftermath of his election, there may have been a 'secret society' of folks within the Department of Justice and the FBI, to include Page and Strzok, that would be working against him."

Some alleged a cover-up when the department admitted Monday that the FBI had lost Strzok-Page text messages between December 14, 2016 and May 17, 2017 — a crucial period for the Russia collusion probe -- due to technical issues.

In a statement, senior House Republicans called the thousands of text messages they did receive "extremely troubling."

"Rather than clearing up prior FBI and DOJ actions, these recently produced documents cause us to further question the credibility and objectivity of certain officials at the FBI."

But House Democrats lashed back.

"Republicans are now attacking the FBI in order to undermine Special Counsel Mueller and protect President Trump," Jerrold Nadler and two other senior Democrats said.

"These Republican attacks show their desperation at the fact that Mueller already has obtained two guilty pleas, two indictments, and at least two cooperating witnesses," they said in a statement.