No election promise has haunted a leader in this parliament as much as that made by Nick Clegg over tuition fees. It is an error that the Deputy PM has no intention of repeating this time round - but his stance has gifted Labour an attack line. After the party calculated that fees would need to rise from £9,000 to £11,500 to cover an anticipated £1.5bn cut in the higher education budget by the Conservatives, Yvette Cooper challenged Clegg five times on the Andrew Marr Show to rule out any increase. The Deputy PM responded by trying to subject to nursery and school funding, allowing Cooper to claim victory: "So you won't rule out a further increase in tuition fees ... We'll protect the education budget but we will also cut tuition fees [to £6,000]".

But while Clegg has avoided any red line over tuition fees, he has set out six others: an £8bn increase in the NHS budget, a £2.5bn increase in education funding, guaranteed public sector pay rises, the elimination of the structural deficit by 2017-18, the rejection of £12bn of welfare cuts (as proposed by the Tories) and the protection of the environment through new green laws. As their polling ratings remain becalmed, Lib Dem strategists are emphasising these in an attempt to show voters what they will gain if the party wins enough MPs to form another coalition.

Significantly, Clegg's red lines do not include the avoidance of an in/out EU referendum. Under questioning from Marr, he made it clear that he would allow a vote to take place (without which David Cameron has said he will not remain prime minister) in return for concessions in areas such as welfare cuts. That in turn could lead - and has led - to a split with Vince Cable, who remains opposed to an automatic referendum. Clegg's willingness to risk the UK's EU membership would be one of the biggest obstacles to winning the approval of his MPs and party activists for another deal with the Conservatives.