"The president is floating trial balloons about derailing the Mueller investigation", said Charles Schumer, D-N.Y., the Senate's top Democrat, adding that Republicans have an obligation to "make it clear that firing Mueller is a red line for our democracy that can not be crossed".
"Does anyone think this is fair?"
"The Mueller probe should never have been started in that there was no collusion and there was no crime", he tweeted Saturday.
Trump ignored noting that at least at one point Mueller was a registered Republican voter and is generally viewed in Washington as an apolitical prosecutor, whose investigation of the Trump campaign is supported by Democrats and key Republicans who voiced their support on Sunday news shows for Mueller's handling of the probe.
Mr McCabe documented his interactions with Mr Trump in a series of memos, and those memos could play into Mr Mueller's probe.
Mr McCabe's firing also adds fresh fuel to Mr Mueller's probe. "I have terminated the employment of Andrew McCabe effective immediately".
Dowd told CNN he was speaking on his own behalf, although he had earlier told the Daily Beast, which first reported the statement, that he was speaking on behalf of the President.
Continuing to attack the investigation into Russian interference in the 2016 elections, and any links to his campaign, President Donald Trump on Sunday went on Twitter to directly question the veracity of former top officials of the FBI, accusing them of lying, and making up information to use against him in the Special Counsel's Russia investigation.
He also claimed that Mr Mueller's team of prosecutors has "13 hardened Democrats, some big Crooked Hillary supporters". "Another Dem recently added", Trump said.
It is true that some Mueller investigators have contributed to Democratic political candidates, including Trump's 2016 opponent, Hillary Clinton.
But Mueller defenders say he, and more broadly the FBI, have safeguards in place to avoid letting any investigation be tainted by politics.
After his firing, McCabe said he was the victim of a Trump administration "war" against the Federal Bureau of Investigation and the special counsel.
It seems to me increasingly possible that, despite its having seemed for some months that Trump was just going to flail around helplessly without crossing any major Rubicon or trip-wire, Trump will ultimately go for broke on shutting down the investigation.
Afterward, McCabe said in a statement the firing was "driven by the president himself, to remove me from my position, destroy my reputation, and possibly strip me of a pension that I worked 21 years to earn".
"Any attempt by the President to obstruct or remove the special counsel would create a constitutional crisis and represent an attack on the core American principle that nobody, including the President of the United States, is above the law", he said.
Rep. Trey Gowdy, R-South Carolina, who sits on the House Intelligence Committee, appeared on "Fox News Sunday" to address Mr. Trump personally.
Trump has railed against leaks to the media but has had little success stopping them.
Trump's reaction: "He never took notes when he was with me. Same with lying James Comey".
So now we can see Trump flexing his powers, as President, to strike at enemies.
In an interview with ABC News, he pushed back, claiming he's been caught up in a politically-motivated effort to hurt the Russian Federation investigation.