Winston Peters has promised if he and David Seymour ever got in the ring, he'd put the ACT Party leader in an ambulance with a single punch.
The NZ First leader was responding to a tweet from Seymour on Monday morning, which took aim at the 75-year-old's age and anti-immigration rhetoric.
"Winston Peters' swansong promise to slash immigration is tragic," Seymour, 37, tweeted.
"Peters himself will soon be retired and will require a care worker to help him get dressed and go for a walk. He'll discover that such facilities can't function without migrant workers."
Peters quoted Seymour's tweet to defend his record and boast about his physical fitness.
"I've spent much of my career respecting and working for retirees. You seem to want to euthanise them. As for your nasty comments about my physical - I reckon you'd last ten seconds in the ring with me.
"There'd be three hits - you hitting me, me hitting you, and the ambulance hitting 100. Thank your lucky stars I'm not into physical violence."
Seymour is behind the End of Life Choice Act, which Kiwis will vote on at this year's election in a referendum.
Peters recently took a fortnight off after eating raw meat and falling sick.
Seymour later appeared to try and make peace with Peters.
"Someone's been hit where it hurts - with the truth," he wrote. "P.S. Thank you for voting for the End of Life Choice Act. And there's no shame in being retired. I hope you enjoy it."
Asked on Saturday how many elections he had left in him, Peters pointed to Mahathir Mohamad - who recently retired as Prime Minister of Malaysia at the age of 95.
"I do not intend to duplicate that, but thanks for the vote of confidence," he told TVNZ.
Peters has long portrayed himself as sticking up for the elderly. In 2011, an accidental recording of then-Prime Minister Sir John Key caught him telling then-ACT Party leader John Banks Peters' supporters were "dying off".
Victoria University political scientist Bryce Edwards told The AM Show it's likely the end of the road for Peters, with the party polling abysmally and new National Party leader Judith Collins likely to hoover up the conservative vote.
"I think it is time to start writing off Winston Peters and NZ First."