President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE escalated his long-standing feud with Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (R-Ariz.) on Friday, slamming the retiring senator for trying to force a vote on legislation to protect special counsel Robert Mueller.

“Jeff Flake(y) doesn’t want to protect the Non-Senate confirmed Special Counsel, he wants to protect his future after being unelectable in Arizona for the 'crime' of doing a terrible job! A weak and ineffective guy!” Trump tweeted Friday.

Jeff Flake(y) doesn’t want to protect the Non-Senate confirmed Special Counsel, he wants to protect his future after being unelectable in Arizona for the “crime” of doing a terrible job! A weak and ineffective guy! — Donald J. Trump (@realDonaldTrump) November 9, 2018

Flake, on Thursday, said he and Sen. Christopher Coons Christopher (Chris) Andrew CoonsMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Del.) would go to the Senate floor next week to try to get a deal on advancing the legislation.

The Senate Judiciary Committee last year passed a bill to protect Mueller — or any other special counsel — in the event he is fired, but that legislation stalled amid opposition from GOP leadership. Any new legislation would likely meet the same fate with the Republicans’ expanded majority in the upper chamber.

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The president has long criticized Mueller's investigation into possible collusion between the Trump campaign and Russia, denouncing it as a “witch hunt.”

Mueller and the future of his special counsel investigation were thrown into question on Wednesday when Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE announced he was resigning at Trump's request. Soon after Sessions's resignation, Trump announced he would be replaced with acting Attorney General Matthew Whitaker.

Whitaker has been the focus of scrutiny for his history of public comments criticizing the special counsel investigation. The announcement on Wednesday of his appointment to oversee the special counsel's investigation sparked immediate calls from Democrats for Whitaker to recuse himself and for Congress to pass legislation protecting Mueller.

Thousands around the U.S. participated in protests Thursday evening to "Protect Mueller."

Trump’s comments come two days after he took credit for Flake’s retirement.

“I retired him. I’m very proud of it, I did the country a great service," Trump said during a lengthy press conference at the White House.

Flake, who announced in October 2017 that he would retire after this year, has acknowledged that his vocal criticism of the president’s rhetoric would likely make winning a GOP primary to seek reelection untenable.

“The bottom line is if I were to run a campaign that I could be proud of and where I didn’t have to cozy up to the president and his positions or his behavior, I could not win in a Republican primary, that’s the bottom line," Flake said on MSNBC’s “Morning Joe” after announcing his retirement.

Flake has since remained a public critic of Trump’s rhetoric.