Gordon Brown has called for an official investigation into George Osborne's dealings with a Russian billionaire and his motive may be revenge.
For months the Prime Minister has suffered barbed insults about his personality from George Osborne, the shadow chancellor.
In the Commons today, Mr Brown finally got his own back. Right at the end of prime minister's questions he said the "yachtgate" allegations that Mr Osborne solicited an illegal donation from a Russian oligarch were "serious" and merited an official investigation.
Even though there is no sign of any criminal offence being committed - the Conservatives decided not to accept the donation and no money changed hands - Mr Brown knew his intervention would pile the pressure on the Tories.
It kept the Tory donations row high up in the headlines, particularly on a day when the Prime Minister finally admitted what economists have been saying for weeks - that recession has arrived.
Mr Brown could not resist the temptation to take Mr Osborne down a peg or two in front of delighted Labour MPs.
Unlike Lord Mandelson, who is happy to rub shoulders with senior Tories at gatherings of the rich and powerful, the Prime Minister has a visceral, tribal dislike of Conservatives.
And David Cameron and George Osborne are the two Conservatives he probably dislikes most of all, partly because he regards them as "Tory toffs" who adopt a patronising attitude towards him.
Mr Osborne has made no secret of the fact that he believes Mr Brown is a dysfunctional politician, deeply flawed personally and unable to extend even the normal political courtesies to his political opponents.
He claimed that when they shadowed each other across the despatch box - before Tony Blair quit as Prime Minister - Mr Brown refused to talk to him.
Relations reached rock bottom when Mr Osborne refused to pair off with Mr Brown for an important Commons vote in late 2005. Mr Brown slammed the phone down - and was forced to cut an overseas trip short and fly home to vote.
Mr Osborne has described Mr Brown as "weak", "brutal", "unpleasant", "a phoney" and "a failure" and was rebuked in the Commons for declaring that "Brown will make an effing awful Prime Minister". He was even criticised by a charity for appearing to suggest Mr Brown could be "faintly autistic".
Having suffered months of turmoil and humiliation over Labour's "loans for peerages" scandal - which eventually fizzled out without any charges - Mr Brown clearly wants the Conservatives to suffer too.
But despite a clutch of Labour donors donning ermine, Scotland Yard's finest could not uncover sufficient evidence to stand up in court. It is questionable whether they would want to start delving into what was or was not said in private gatherings in a villa on Corfu and a luxury yacht moored offshore.
Nor is Lord Mandelson likely to relish the prospect of an official inquiry. It would throw the spotlight on his friendship with the billionaire aluminium oligarch, Oleg Deripaska, who is at the centre of the controversy.
Tony Wright, Labour chairman of the public administration select committee - which probed the cash for honours affair - today dismissed calls for an investigation.
There had not been any corruption or law breaking. But there had been a "massive misjudgment" by Mr Osborne, who should have walked away immediately there was any suggestion of a political donation from Mr Deripaska, he said.
And Mr Osborne had broken the rules of the club of the rich and powerful by leaking details of Mr Mandelson's private comments about Gordon Brown as well as his contacts with the Russian tycoon.
Dr Wright told the Press Association Mr Osborne had received his "comeuppance". The lesson of the affair was "don't mess with Mandy".
Once a Tory high flyer, lauded by his party for the tax promises which panicked Mr Brown into dropping plans for a snap general election last year, Mr Osborne is now badly wounded. Mr Cameron is standing by him - but his credibility has been damaged.