Nick Clegg now faces 10 days that are likely to define his political career. Can he maintain the surge that suggests the Liberal Democrats might win more votes than Labour for the first time since 1918, giving him the best opportunity any Liberal leader has had of putting electoral reform on the political agenda? Or does the surge slip away, perhaps still leaving his party with a credible third place, but once again in a relatively weak bargaining position in the Commons?
Lib Dem support has slipped in the wake of the more evenly contested second leaders' debate. The party's average poll rating now stands at 29 per cent, down a point on all polls conducted between the first and second leaders' debates.
True, one weekend poll from Mori gave the party only 23 per cent. But this contained unusually few people who said they voted Lib Dem in 2005. Unlike many other pollsters, Mori does not weighting its data by how people said they voted last time, and as a result appears to have fallen victim to a rogue sample.
Despite that slippage, the Lib Dems are still ahead of Labour, whose average rating has held steady on 27 per cent. So the Lib Dems still have a good chance of coming second in the popular vote, cracking open the mould of the post-war, two-party system. But that two point gap can hardly be described as comfortable.
Considerable evidence points to the Lib Dem vote being relatively soft. Most polls find that its current supporters are far more likely than their Tory and Labour counterparts to say they might change their minds. The party is also doing relatively well among those who did not vote last time – and they may well need a disproportionate amount of cajoling to make it to the polls this time too.
There are also signs that the prospect of a hung parliament may not be greeted with enthusiasm by many voters. As many as 54 per cent told ComRes that a hung parliament would put the economic recovery at risk, while BPIX reported that 55 per cent believe it would result in weak government. More importantly, even Lib Dem supporters appear to be ambivalent about such an outcome.
Mr Clegg seems to be aware of some of this concern. Yesterday he addressed one point directly – that it might enable Gordon Brown to cling to power even though Labour came third in votes. In fact, a two point increase in Tory support to 34 per cent means that prospect now looks more remote anyway.
On the conventional calculations, the latest poll figures point to a Commons with 272 Tory MPs, 255 Labour, and 91 Lib Dems, while other parties provide the remaining 32. The Tories would still be 52 seats short of a majority, and would be compelled to come to some understanding with a Lib Dem party that, by virtue of its second place in votes, would feel emboldened to strike a hard bargain, including on electoral reform.
But if Lib Dem support were to slip behind Labour, the party's moral authority and political clout would be much weaker. On its own, such a development would not be enough to give Mr Cameron a majority – his current seven point lead over Labour is insufficient for that. But he would claim to be the moral victor – from whom a diminished Mr Clegg might only be able to secure relatively few crumbs.
John Curtice is Professor of Politics, Strathclyde UniversityReuse content