"He concerns me," Corker told the Times. "He would have to concern anyone who cares about our nation." He added, "I know for a fact that every single day at the White House, it's a situation of [senior administration officials] trying to contain him."
The explosive comments not only breach what had been one of Trump's few personal relationships on Capitol Hill, but also jeopardize the president's legislative priorities. As chairman of the foreign relations panel, Corker (Tenn.) will help determine the future of the Iran nuclear deal, and his support will be critical in passing broad tax cuts.
Scenes from Trump’s second six months in office
Trump alleged Sunday morning in a Twitter tirade that Corker recently "begged" him for his endorsement, did not receive it and decided to retire because he "didn't have the guts" to run for reelection next year.
Corker tweeted a biting retort: "It's a shame the White House has become an adult day care center. Someone obviously missed their shift this morning."
Meanwhile, Corker's chief of staff, Todd Womack, denied Trump's characterization of his private conversations with the senator, who announced last month that he plans to retire and not seek reelection in 2018.
Trump's outburst comes after Corker made headlines last week when he starkly suggested that the administration's national security team provides the president with badly needed adult supervision. Corker told reporters that Secretary of State Rex Tillerson, Defense Secretary Jim Mattis and White House Chief of Staff John F. Kelly "are those people that help separate our country from chaos."
Trump, who has little tolerance for public criticism and prides himself on counterpunching those who cross him, took to Twitter on Sunday to attack Corker.
Trump tweeted: "Senator Bob Corker 'begged' me to endorse him for re-election in Tennessee. I said 'NO' and he dropped out (said he could not win without . . . my endorsement). He also wanted to be Secretary of State, I said 'NO THANKS.' He is also largely responsible for the horrendous Iran Deal! Hence, I would fully expect Corker to be a negative voice and stand in the way of our great agenda. Didn't have the guts to run!"
Womack said Trump has repeatedly offered to support Corker, and as recently as last week asked the senator to change his mind and run for reelection.
"The president called Senator Corker on Monday afternoon and asked him to reconsider his decision not to seek reelection and reaffirmed that he would have endorsed him, as he has said many times," Womack said in a statement.
Apparently unwilling to let Corker's "adult day care center" barb be the last word, Trump tweeted Sunday afternoon that Corker was an ineffective senator and could not "get the job done."
"Bob Corker gave us the Iran Deal, & that's about it," Trump tweeted. "We need HealthCare, we need Tax Cuts/Reform, we need people that can get the job done!"
In a 25-minute telephone interview with the Times, Corker questioned Trump's fitness for office and said that the president's recklessness is endangering the country — an assessment he said was shared by most other Senate Republicans.
"Look, except for a few people, the vast majority of our caucus understands what we're dealing with here," Corker told the Times. He added, "Of course, they understand the volatility that we're dealing with and the tremendous amount of work that it takes by people around him to keep him in the middle of the road."
Corker, who is one of Tillerson's few confidants and staunch defenders in Washington, criticized a Trump tweet last weekend that undercut Tillerson's negotiations with North Korea.
A lot of people think that there is some kind of 'good cop, bad cop' act underway, but that's just not true," Corker told the Times. He added, "I know [Trump] has hurt, in several instances, he's hurt us as it relates to negotiations that were underway by tweeting things out."
The squabble comes just days before Trump is expected to "decertify" the Iranian nuclear pact reached with world powers in 2015 and punt to Congress a decision about whether to restore sanctions against Iran. This would be the first in a series of highly orchestrated steps that White House, State Department and congressional officials — primarily Corker, as chairman of the Foreign Relations Committee — have been planning for months.
Corker has been one of Tillerson's few allies and staunch defenders in Washington, working closely on such issues as toughening sanctions on Russia and engaging North Korea diplomatically — two issues on which Trump has disagreed with Corker.
Corker also looks to play a key role in the upcoming debate over taxes. One of the Senate's most committed deficit hawks and outspoken members on budgetary issues, Corker already has expressed concerns with the Trump administration's proposal on tax cuts, and his vote will be key to any deal getting done.
Trump's attack also highlighted his increasingly strained relationship with Senate Republicans, who Trump feels have failed to deliver on his agenda.
In recent months, Trump has also gone after Senate Majority Leader Mitch McConnell (Ky.) and Sens. Lindsey O. Graham (S.C.), John McCain (Ariz.) and Jeff Flake (Ariz.) with cutting and sometimes personal insults.
Republican lawmakers and operatives have voiced exasperation that Trump is spending his time attacking senators he will need as allies if he hopes to sign any signature legislation.
"The Corker-Trump side show, which is all it is, is great if you love reality TV," GOP strategist Rick Tyler said. "We can stay tuned for the next episode. But it doesn't get you anywhere. It may feel satisfying, but it is meaningless in the context of Republicans proving that they can be a governing majority."
Corker was a prominent supporter of Trump's 2016 campaign and one of the few Republicans with gravitas willing to embrace the reality television star's candidacy before he won the GOP nomination.
Corker publicly praised Trump's first major foreign policy speech in April 2016, at a time during the primaries when most other party elders were shunning Trump. Corker also helped tutor Trump on foreign affairs, and he in turn considered the senator as a possible running mate and secretary of state.
Both Trump and Corker entered politics after careers as business executives, both in real estate and construction, and their shared backgrounds gave them a level of mutual understanding at a time when few in Congress can claim to understand the president's motivations. Corker was one of only a few senators to develop a personal relationship with Trump and his family.
Known for his blunt commentary, Corker had long softened any public criticism of Trump or his administration with carefully worded praise. But that changed this past summer, as tensions between the men flared.
In August, Corker criticized Trump's handling of the deadly white-supremacist rally in Charlottesville, saying, "The president has not yet been able to demonstrate the stability nor some of the competence that he needs to demonstrate in order to be successful."
Then, as now, Corker became a subject of Trump's ire. In response to the senator's "stability" and "competence" comments, Trump tweeted, "Tennessee not happy!"
Trump later gave Corker grief about the comment during a meeting in the Oval Office, according to a Republican congressional official who spoke on the condition of anonymity because of the sensitivity of the private conversations.
But the episode did not seem to derail what was generally a good-natured relationship between the two. When Corker later called Trump to tell him that he had decided to retire — a decision Corker made on his 65th birthday, Aug. 24 — the president expressed disappointment, the congressional official said.
More recently, Trump has fumed over Corker's tangential connection to last month's Alabama Senate Republican primary runoff election, in which Trump's favored candidate, Sen. Luther Strange, was defeated by former judge Roy Moore, who enjoys the backing of many of Trump's supporters in the state.
Corker urged Trump to visit Alabama and campaign alongside Strange in the closing days of the runoff campaign, and the president now partly blames Corker for encouraging him to get involved in a contest that has hurt his political standing, according to a person close to Trump and familiar with the dynamics.
Mick Mulvaney, director of the Office of Management and Budget, said Sunday that he thinks Corker feels free to speak his mind now that he is not seeking reelection.
"I think it's going to be fun to work with him, especially now that he's announced that he's not running for reelection, because I think it sort of unleashes him to do whatever and say whatever he wants to say," Mulvaney said on NBC's "Meet the Press."
Robert Costa contributed to this report.