PM to argue reform is 'opposite of what we need' but deputy will say change is good for our democracy
David Cameron and Nick Clegg will go head-to-head over Britain's voting system with Cameron warning AV would lead to further political horsetrading, and Clegg countering that first-past-the-post fosters distrust in politics. In carefully choreographed speeches, following the referendum on 5 May being given royal assent, the two men will start a delicate operation to set out views on voting reform passionately held by their respective parties without damaging the coalition's stability.
One source described it "as a temporary and limited undocking". Cameron will speak first in London followed two hours later by Clegg in Leeds.
Cameron is expected to be the more prominent campaigner in the coming week after polling showing Clegg is the best poster boy for the no campaign.
The prime minister will say: "In the next 11 weeks the debate over AV is going to heat up right across the country. Throughout this time I'll be making my case loud and clear."
He will claim the alternative vote (AV) – the system advocated by Clegg – is "the precise opposite of what we need right now". His uncompromising attack will reassure Tory activists worried that he might be privately content with a yes victory, either to protect a vulnerable Clegg from a Liberal Democrat backlash or because he believes AV could make a second Lib-Con coalition more likely after a 2015 election.
Cameron is being urged to ensure the Conservative party donates to the cash-strapped no campaign. Some Conservatives also want the party to put the fact that the Liberal Democrats will benefit centre stage, but Cameron has declined. Literature emerging from Central Office focuses solely on specific drawbacks of AV such as the third-placed party winning, or the system's lack of use round the world. It does not refer to the Liberal Democrats.
The official no campaign by contrast plans to make Clegg the centrepiece of its campaign.
Cameron faces the delicate task of arguing that AV is more likely to lead to coalition governments without criticising his own coalition with the Liberal Democrats. He will claim that coalitions can bring parties together in the national interest, but that under AV "hung parliaments will become commonplace".
He will add: "Where there are more hung parliaments there will be more haggling and horsetrading between parliaments – both before and after elections."
He will add "there will be gamemanship between parties in different constituencies as they try to stitch up second preference votes".
He will claim that AV at the last election, could have kept Gordon Brown as prime minister since AV will lead to second-choice government.
Clegg will give a restrained case for AV, eschewing the rhetoric he normally deploys against constitutional conservatives. He will say: "First past the post is out of date and it is at the heart of so many of the reasons that people don't trust in or care about politics. It means most MPs are elected without the support of most of the people they are supposed to represent. It means millions of votes make no difference whatsoever. It means millions of voices going unheard. It makes it easy for MPs to get complacent and lazy. First past the post is not working and it's time to get rid of it.
"Under the alternative vote, politicians will need to aim to get half of their constituents to choose them. That means they will have to work harder to appeal to more people than before. It means they will have to reach out to people who were ignored under first past the post. It means they will no longer be able to rely on just their core supporters and ignore everyone else. They will be more legitimate and will carry a stronger mandate from a broader range of people. That can only be good for our democracy.
"Under the alternative vote, there will be fewer MPs with jobs for life in safe seats. That means people whose voices have been ignored will be listened to again. It means that parties will have to compete for votes in every corner of the country and not just those few marginal seats.
"Under the Alternative Vote, there need never be another wasted vote. That means you can use your vote positively. It means you can feel confident voting for the person or party you want to win and not have to think tactically about who can. That can only be good for our democracy."