The unexpectedly bitter battle between the Conservatives and Liberal Democrats over electoral reform has left little space in the debate for Labour.
Ed Miliband's party is ahead in the opinion polls and should gain 1,000 seats in the English council elections. But most headlines after 5 May will probably be about the fallout from the referendum on the alternative vote (AV). And the polls suggest that Alex Salmond's Scottish National Party could deny Labour the expected prize of forming the next Scottish government.
Mr Cameron's surprisingly high profile in the referendum campaign – the opposite of what he agreed with Nick Clegg beforehand – has made Tory supporters aware of the plebiscite and boosted the No camp's poll ratings.
Most debate has focused on whether the Liberal Democrats would walk out of the Coalition before 2015, when the next general election is due. But Mr Cameron's blitz has prompted speculation that if he wins the referendum, he might pull the plug on the Coalition and call a "dump, cut and run" election as early as this autumn.
It seems far-fetched, but there would be a lot of attractions. The Tories have plenty of money, unlike Labour and the Liberal Democrats. The Government's spending cuts have not yet sunk in and the Tories remain steady in the polls. Labour is plainly not ready for an election. It is blamed by the public for the deficit. Ed Miliband has not had time to establish himself and his policies are a "blank sheet".
Although an early election probably won't happen, the very prospect ought to encourage Labour supporters to vote Yes to AV. Yet Mr Miliband cannot mobilise his party's natural supporters in the way Mr Cameron has done.
With the Tories and Liberal Democrats at each other's throats, Labour sympathisers cannot simply vote against the Coalition. Many will be tempted to "kick Clegg", despite the Labour leader's plea in The Independent last week not to make the referendum one on the Deputy Prime Minister.
The Yes campaign has been hampered by Labour's divisions. It is split down the middle, with 125 of its 258 MPs backing a No vote. It was ever thus. Although the party swallowed Gordon Brown's last-minute offer of an AV referendum in the party's manifesto last year, that was only because the election was imminent.
Mr Miliband has a difficult hand to play. He backs AV and wants to reach out to the Liberal Democrats, knowing his party may need them to form a government again. But Labour's split has limited his room for manoeuvre.
He decided to campaign for a Yes vote but to refuse to share a platform with Mr Clegg, a hate figure for many Labour folk. Yet that only advertises another problem for the Yes camp.
A switch to AV might be only a small change, but the implications of the referendum for all the party leaders are huge.