Donald Trump's dig at Australia's man in Washington has fanned a political firestorm on home soil.
The US presidential nominee offered a withering character assessment of Kevin Rudd during a television interview.
"I don't know much about him," Mr Trump told London's GB News.
"I heard he was a little bit nasty.
"I hear he's not the brightest bulb ... if he's at all hostile, he will not be there long."
Mr Rudd was a vocal critic of the former Republican president before he was appointed US ambassador.
He described Mr Trump as a "traitor to the West" and "the most destructive president in history" who "drags America and democracy through the mud".
The opposition was quick to pounce on Mr Trump's comments during Question Time on Wednesday.
The prime minister was not having it, describing the line of questioning as a "cheap shot" and accusing the coalition of politicising the ambassador role.
"They decide to politicise Australia's representative in Washington DC," Anthony Albanese told parliament.
He said that was never done by Labor when former Liberal politicians, including Arthur Sinodinos and Joe Hockey were appointed to that position.
"Because one of the things that I have never seen happen before in 28 years in this chamber is an attempt to politicise Australia's representative overseas in an important nation such as that," Mr Albanese said.
Opposition Leader Peter Dutton said Mr Rudd needed to repair the relationship with the former US president.
"It is in our national interest to make sure that the Ambassador to the United States, our most important strategic alliance partner, is successful and we will do everything we can to support him," he said.
The US presidential election in November is a rematch between Democrat President Joe Biden and Mr Trump.
Mr Biden's approval rating has continued to slip, while the former president is contending with multiple criminal charges.
Nationals leader David Littleproud warned Mr Rudd's past comments could have consequences for Australia's alliances and defence deals if Mr Trump won the election.
"We're going to have a very difficult time in negotiating with the new administration, particularly with things as important as AUKUS," he told Sky News.
But Foreign Minister Penny Wong defended her former colleague and said Australia would keep Mr Rudd as ambassador.
"Mr Rudd is a very effective ambassador," she told reporters.
"His experience and skills mean he will be able to work closely with whomever is elected by the American people."
