US President Donald Trump has been warned by fellow Republicans against interfering in special counsel Robert Mueller's investigations.
It came after he attacked Mr Mueller's inquiry into Russian interference in the 2016 US presidential election.
In tweets at the weekend, Mr Trump reiterated that there had been "no collusion" between his team and Russia and called the probe a "witch hunt".
He added that it was dominated by "hardened Democrats".
Mr Mueller, a highly regarded former head of the FBI, is a Republican.
Republican Senator Lindsey Graham said Mr Mueller should be allowed to proceed without interference, and that many Republicans shared his view.
He also warned Mr Trump against any attempt to dismiss Mr Mueller.
"If he tried to do that, that would be the beginning of the end of his presidency, because we're a rule of law nation," Mr Graham said.
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Republican Senator Jeff Flake, a frequent Trump critic, said it appeared the president's latest comments seemed to be preparing the ground for the firing of Mr Mueller.
"I don't know what the designs are on Mueller, but it seems to be building toward that, and I just hope it doesn't go there... We can't in Congress accept that," he told CNN.
"I'm just puzzled by why the White House is going so hard at this, other than that they're very afraid of what might come out."
AshLee Strong, a spokeswoman for the Republican Speaker of the House of Representatives, Paul Ryan, said: "As the Speaker has always said, Mr Mueller and his team should be able to do their job."
Senate Democratic Leader Charles Schumer accused Mr Trump of "floating trial balloons about derailing" the investigation.
"Our Republican colleagues, particularly the leadership, have an obligation to our country to stand up now and make it clear that firing Mueller is a red line for our democracy that cannot be crossed," he said in a statement.
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