Vote referendum: Clegg v Cameron
David Cameron and Nick Clegg can sometimes appear joined at the hip but among the issues on which they fundamentally disagree is the way Britain elects its MPs.
Britain will go to the polls on 5 May to decide whether to keep the first-past-the post voting system at Westminster elections or change to the Alternative Vote method, in which voters rank their choices in order of preference.
The prime minister and deputy prime minister have both now set out where they stand in separate speeches. Here is what they had to say - followed by an analysis of how they said it.
HOW DID THEY DO?...
Anyone hoping for a punch-up in the Downing Street rose garden would have been disappointed.
They were in different parts of the country for a start.
But David Cameron and Nick Clegg's speeches on changing the electoral system did mark a significant moment in the coalition government's history.
It was the first time the prime minister and deputy prime minister - who so often seem in agreement - had made speeches arguing for diametrically opposing positions.
Both men stressed from the outset that the coalition would remain intact whatever the outcome of 5 May's vote.
But, joked Mr Cameron, "on this one, I don't agree with Nick," echoing last year's election catch phrase.
Mr Cameron struck a more populist tone in his speech - he name checked Usain Bolt at one point, arguing that you would not settle for second preferences at the Olympic Games so why should it happen at an election?
Moral high ground
The Conservative leader was trying to come across as a hard-headed pragmatist - few other countries used the Alternative Vote and the current system was brutally simple and easy to understand, so why go to the trouble and expense of changing it for a system that nobody really wanted?
He also argued that first-past-the-post made it easier for voters to kick governments out - recalling, with a smile, the sight of cabinet colleague Ken Clarke, now chairman of the No2AV campaign, loading his possessions into a removal van after the Conservatives were ejected in 1997.
Mr Clegg, on the other hand, attempted to take the moral high ground - returning to the sort of idealistic rhetoric we used to hear from him before the general election.
He claimed changing the voting system was a crucial part of cleaning up politics - making repeated references in his speech to 2009's expenses scandal.
He even argued that the most corrupt MPs had tended to be in safe seats - something he says will be a thing of the past with AV.
In one sense, Mr Cameron is in a more difficult position because a big part of his argument is that AV would lead to more hung parliaments and deals being made behind closed doors with both sides ditching their manifesto pledges.
Which some would argue is a pretty good description of how the coalition formed.
Mr Cameron, in the question session afterwards, argued that the coalition was a necessary response to the crisis the country was in and what people had voted for - but under AV it would happen a lot more. It would, he argued, come to be seen as the "politician's stitch-up system".
Meanwhile, Mr Clegg - in an echo of the sort of difficulties he has faced over his pre-election pledge on tuition fees - is having to deal with claims he has done a U-turn on AV.
Before the election, the Lib Dem leader described it as a "miserable little compromise". David Cameron referred to this in his speech - the closest either of them came to mud slinging.
But there were a few signs of cracks in their carefully choreographed show of "grown-up" politics.
Mr Clegg dismissed claims that AV would lead to the introduction of electronic voting machines as "scare stories".
But in his speech, Mr Cameron cited voting machines as one of the many reasons why AV was a bad idea.
In their different ways, they both sounded like party leaders again, rather than coalition partners.
Whether this will start to drive a wedge between them may depend on how often they go into bat for their respective sides, as the campaign develops.