Boris Johnson openly contemplated a forced removal of Danny Alexander from the Treasury today, as he pressed Downing Street into dropping the top rate of tax to 40p.
The comments came after Alexander said he would only allow a reduction in the top rate "over my dead body".
Asked about the comment on the Today programme this morning, Johnson replied: "Who can say what will happen to Danny Alexander? Stranger things have happened at sea."
He added: "I can't believe we're going to go into an election keeping tax rates higher than our competitors".
Alexander is trying to position the Liberal Democrats in the middle between a growing Labour-Tory debate on the appropriate top rate of income tax, amid growing signs that George Osborne and David Cameron are preparing to cut it to 40p.
Cameron was asked to rule out a cut three times during last week's PMQs, but failed to do so.
"The top rate of tax has been an issue of late. Labour wants to take it back up to 50p, I think the 45p rate is the right place to be," Alexander said in an interview with the Daily Mirror.
"I wouldn’t go to cutting below 45p – that would happen over my dead body. It’s better to say we are going to stick where we are."
The chief secretary to the Treasury also hit out at those in his own party who say he has 'gone native' and effectively become a Tory.
"If that's what people think about me, then they are wrong. I am Liberal Democrat - full stop, end of story," he said.
"We must get our fair share of credit for the economic recovery.
"The economic plan is not a Tory plan, it's a coalition plan. There’s as much Lib Dem thinking as Tory thinking. It's as much Danny Alexander's economic plan as it is George Osborne's."
But amid the political positioning Alexander also held out an olive branch to Labour, who he may need to enter coalition negotiations with after the 2015 general election.
"I play cricket with Ed [Balls]," he said.
"I have opened the bowling while he was behind the stumps and he didn't let anything past him. So I would endorse him as a wicketkeeper."
He also tried to put some clear blue water between his position on future deficit reduction drives and that of the chancellor.
"George Osborne wants the remaining deficit reduction done all through spending cuts. I don't agree," he said.
"The Lib Dems will fight the election promising higher taxes on the richest. This includes a mansion tax, slashing tax relief on pensions and on capital gains taxes. The wealthy must contribute."
Chris Leslie, Labour's shadow chief secretary to the Treasury, said: "Danny Alexander seems to have forgotten that just ten months ago he cut the top rate of tax for the top one per cent of earners. And that was after saying anybody who wanted to do this was living in 'cloud cuckoo land'.
"After cutting taxes for the richest while breaking their election promise by raising VAT on everybody else, nobody will believe a word the Lib Dems say on tax."
Osborne is eyeing an extra £25 billion in spending cuts after the election, much of it through further attacks on welfare.