Republican Senator Jeff Flake took aim at US President Donald Trump on Tuesday as Flake announced that he would not run for re-election in 2018.

"There are times we must risk our careers. Now is such a time," the Arizona senator said in a speech on the Senate floor. 

He criticized the path that the Republican Party has taken under Trump, decrying the "flagrant disregard of truth and decency" that is dragging down politics.

"We must stop pretending that the degradation of politics and the conduct of some in our executive branch are normal. They are not normal," the senator said.

White House press secretary Sarah Huckabee Sanders said Flake's decision not to seek re-election was "probably a good move," arguing that he lacks enough support from voters in Arizona.

Flake, who has served in the Senate since 2013, has been an outspoken critic of the Trump administration. He became the second Republican to announce he would not be running for re-election next year, joining Tennessee Senator Bob Corker.

Trump's war of words

Flake's announcement follows an escalation in the public feud between Corker and Trump, who traded insults on Twitter and on television on Tuesday. 

"The president has great difficulty with the truth on many issues," Corker told CNN during an interview at the Capitol.

Corker, who chairs the Foreign Relations Committee and previously supported the president, said Trump has damaged the standing of the US around the world.

"I don't know why he lowers himself to such a low, low standard and debases our country in the way that he does, but he does," Corker said, noting that he wouldn't support Trump's presidential bid again. 

"He has proven himself unable to rise to the occasion" as president, Corker added.

Read more: Trump to 'let Obamacare fail,' focus shifts to fiscal reform

Trading insults

Trump's spat with Corker came after the president attacked the senator in a series of morning tweets, with the discourse between the two ranging from serious criticism to playground insults.

Ostensibly set off by an interview Corker gave to NBC about Trump's tax plan early Tuesday morning, the president wrote: "Bob Corker, who helped President O give us the bad Iran Deal & couldn't get elected dog catcher in Tennessee, is now fighting Tax Cuts."

Shortly thereafter, Corker responded on Twitter, writing: "Same untruths from an utterly untruthful president." He rounded off the post with the hashtag "#AlertTheDaycareStaff."

Corker previously dubbed the White House an "adult day care center" and said that Trump's rhetoric could put the US "on the path to World War III."

The senator's response, combined with his remarks to CNN, prompted another barrage of tweets from Trump, in which he called Corker "incompetent" and said "he doesn't have a clue."

Corker has recently grown more vocal in his criticism of the president. Although a foreign policy specialist, he also is a key player in the tax reform debate and his support will be crucial to passing the plan in the Senate.

Read more: Trump unveils long-awaited tax reform plan

Trump has also publicly feuded with several other members of his own party, including Senator John McCain and Senate Majority Leader Mitch McConnell.

rs/cmk (AP, AFP, Reuters)