US presidential hopeful Ted Cruz has been called "Lucifer in the flesh" by the former Speaker of the House of Representatives, John Boehner.
Mr Boehner, a fellow Republican, has also reportedly said he will not vote for Mr Cruz if he becomes the nominee.
Their rift dates back to when Mr Cruz led a group of hard-core conservatives to force a government shutdown in 2013, against his party's leadership.
Meanwhile, Mr Boehner has described Donald Trump as a "texting buddy".
He also said they have played golf together for years and that he would vote for the billionaire if he were the Republican nominee, the Stanford Daily reported.
The billionaire is the front-runner on the race for the Republican nomination, ahead of Mr Cruz.
The Texas senator is seen by many Republicans as the only option to prevent Mr Trump from being the party's candidate. Others, however, dispute this, saying he is a divisive figure.
Mr Boehner, who was the most powerful Republican in US politics for a time until he resigned last October, used strong language when he spoke about Mr Cruz during a talk at Stanford University.
"I have Democrat friends and Republican friends. I get along with almost everyone, but I have never worked with a more miserable son of a bitch in my life," he said.
Mr Boehner had once called Mr Cruz a "jackass"
John Boehner tells us how he really feels.
At a time when Ted Cruz is struggling to save his presidential campaign, having a former high-ranking member of the Republican Party compare him to Beelzebub is, shall we say, unhelpful. It further reinforces the perception - hammered time and time again by Donald Trump - that Mr Cruz is too divisive, too abrasive, too unliked to be a successful leader.
The Texas senator likely would counter that he has made the right kind of enemies and the ire of the party establishment is a badge he will proudly wear. Unfortunately for him, however, the party establishment is just about the only thing left keeping his candidacy afloat. He has become the vessel for the #NeverTrump efforts - the last realistic candidate between Mr Trump on the nomination - and that movement is populated by insiders who, in any other situation, would not hesitate to stick a knife in Mr Cruz's back.
It seems Mr Boehner, happy in his retirement from politics, had no such reservations.
Mr Cruz is credited with having a large role in the federal government shutdown in 2013, when Mr Boehner was Speaker of the House.
The Texas senator is seen as having an aggressive posture and considers himself as an anti-establishment politician.
He reacted to Mr Boehner's remarks on Twitter, saying: "Tell me again who will stand up to Washington? Trump, who's Boehner's "texting and golfing buddy," or Carly & me?", he wrote, referring to Carly Fiorina, his pick for vice-president in an eventual candidacy."
When asked about the Democratic front-runner Hillary Clinton, Mr Boehner reportedly impersonated her saying "Oh I'm a woman, vote for me," which received a negative reaction from the crowd.
He later said they had known each other for 25 years and that he finds the former secretary of state to be very accomplished and smart.