The US Republican chairman of the Senate Foreign Relations Committee has warned that President Donald Trump could put the country "on the path to World War Three".
Senator Bob Corker said Mr Trump was treating the presidency like "a reality show".
The retiring Tennessee lawmaker also said White House staff struggled to "contain" Mr Trump.
Mr Corker's comments were a remarkable attack on a sitting president from a member of his own party.
Mr Corker was considered for the job of secretary of state by Mr Trump last year but they have since fallen out.
The senator told the New York Times in a telephone interview on Sunday he was concerned about the president's conduct.
"I don't think he appreciates that when the president of the United States speaks and says the things that he does, the impact that it has around the world, especially in the region that he's addressing," he said.
Mr Corker suggested the president - who has repeatedly baited nuclear-armed North Korea - could put the US on course for "World War Three".
"I don't know why the president tweets out things that are not true," he said. "You know he does it, everyone knows he does it, but he does."
"I know for a fact that every single day at the White House, it's a situation of trying to contain him," Mr Corker told the newspaper.
The ill-tempered exchange began on Sunday morning, when Mr Trump tweeted that Mr Corker had begged the president to endorse him for re-election.
Mr Trump said he refused. He also accused Mr Corker of being "largely responsible for the horrendous Iran Deal".
Mr Corker, who denied the president's account, responded: "It's a shame the White House has become an adult day care center.
"Someone obviously missed their shift this morning."
Mr Trump is this week expected to de-certify the Obama administration's 2015 agreement to curb Iran's development of nuclear weapons, a deal he has repeatedly condemned.
Last month, Mr Corker - who has said the Iran deal should not be torn up - announced he would not seek re-election at next year's mid-term elections.
Mr Corker and Mr Trump previously clashed in August when the senator criticised the president's response to clashes that month between white supremacists and anti-fascist demonstrators in Charlottesville, Virginia.
Mr Trump's attack followed Mr Corker's expression of support last week for Secretary of State Rex Tillerson, who is widely seen as having been sidelined by the president.
Mr Corker said America's top diplomat was "in an incredibly frustrating place", where he "ends up not being supported in the way that I hope a secretary of state would be supported".
On Wednesday Mr Tillerson denied rumours that he was about to resign, amid reports he had referred to the president as a 'moron'.