Leading Democrat Chuck Schumer has said the sacking of Secretary of State Rex Tillerson shows how unstable the Trump administration has become.
The Senate Minority Leader said he hoped CIA Director Mike Pompeo will "turn over a new leaf" as secretary of state and toughen policies toward Russia and its president, Vladimir Putin.
Mr Schumer said after the firing of Mr Tillerson: "The instability of this administration in just about every area weakens America."
Mr Pompeo is President Donald Trump's pick to replace Mr Tillerson, and he would need to be confirmed by the Senate.
But Republicans have just a slim 51-seat majority, and confirmation is not certain.
Mr Schumer said: "If he's confirmed, we hope that Mr Pompeo will turn over a new leaf and will start toughening up our policies towards Russia and Putin."
Senator Bob Corker, the Republican Foreign Relations Committee chairman, said Mr Trump's decision to fire his chief diplomat caught him by surprise.
Mr Corker, who has been Mr Tillerson's most vocal supporter on Capitol Hill, acknowledged "there's been tensions" between Mr Tillerson and Mr Trump.
But he said it had seemed "that they had learned to work together a little bit better" and he did not know what had caused the sacking.
In October, Mr Corker said Mr Tillerson, along with Defence Secretary Jim Mattis and White House Chief of Staff John Kelly, are "those people that help separate our country from chaos".
On Tuesday, Mr Corker said, "I have a lot of faith in Tillerson's judgment. I've shared that with the president many times."
UN Ambassador Nikki Haley hailed Mr Pompeo as a "great decision" for next secretary of state.
Ms Haley, who had a contentious relationship with Mr Tillerson, congratulated Mr Pompeo in a tweet and called him her "friend".
She wrote: "Great decision by the President."
Mike Pompeo, Director of the CIA, will become our new Secretary of State. He will do a fantastic job! Thank you to Rex Tillerson for his service! Gina Haspel will become the new Director of the CIA, and the first woman so chosen. Congratulations to all!— Donald J. Trump (@realDonaldTrump) March 13, 2018
Over months of speculation about Mr Tillerson's future, Ms Haley had been rumoured as a possible successor.
Donald Trump has fired US Secretary of State Rex Tillerson and replaced him with CIA director Mike Pompeo.
The US president said Gina Haspel will become the new CIA director, the first woman chosen for the job.
Mr Trump tweeted that Mr Pompeo, "will do a fantastic job!" and thanked Mr Tillerson for his service.
Mr Tillerson had returned from a shortened trip to Africa hours before Mr Trump's announcement, which offered no explanation for the change.
A White House official said Mr Trump wanted to have a new team in place before upcoming talks with North Korea and various trade negotiations.
There had been rumours throughout most of Mr Tillerson's tenure of friction between Mr Trump and his secretary of state, a former ExxonMobile chief executive.
In October, NBC news reported that Mr Tillerson called the president a "moron," something he never actually denied.
Mr Tillerson continued to insist his relationship with the president was solid and brushed off rumours of strain between them.
He was due to retire from Exxon in March 2017 aged 65 under the company's mandatory retirement policy.
Mr Tillerson was paid $27.3m (€22m) last year, and has accumulated roughly $160m dollars (€129m) in Exxon stock along with $149m (€120.5m) of unvested stock options.