Nick Clegg, in the words of one source, is "terrified" of losing his seat at the election. The Lib Dem leader won't be feeling any less anxious today after Lord Ashcroft's latest poll showed him two points behind Labour in Sheffield Hallam. The opposition, which finished third in 2010, 19,096 votes behind Clegg, has won a remarkable swing of 19.5 per cent.

There are numerous caveats to insert in response: Ashcroft doesn't name candidates in his polling questions (something that tends to hinder incumbents); Labour's lead is within the margin of error and Clegg still has more than a month to win over Tory voters who don't want a Labour MP (the Conservatives are currently on 16 per cent). As his former special adviser Sean Kemp tweeted: "They're going to get a lot of leaflets."

But the possibility that the Deputy Prime Minister could lose his seat - a moment that would make Michael Portillo's defenestration look banal by comparison - is real enough for us to consider the likely political consequences. Perhaps the most significant is that it would make another Conservative-Lib Dem coalition far harder to assemble. After losing their leader, the Lib Dems would likely be led by Vince Cable in the interim, with Tim Farron the most likely permanent replacement. Both men are notably frosty towards the prospect of another partnership with the Tories. Recently asked about another coalition, Cable replied: "I am not ruling that out but I am not pushing for it." Farron has regularly argued that the Lib Dems should not seek government at any cost and has suggested that a minority government may be preferable. And having lost their leader and perhaps more than half of their MPs, would their party colleagues disagree with them?

The Lib Dem support that the Tories would almost certainly need to form another government if they fall short of a majority may not be available if Clegg loses his seat. It's for this reason that Cameron is privately exercised by the prospect of his deputy being ousted (with the Tories accused of soft-pedaling in the seat). In the absence of a coalition with the Lib Dems, he would find it far harder to remain prime minister and, if he does, to pass significant legislation.

One defining question is whether Cable and Farron would be prepared to accept the Conservative demand of an in/out EU referendum in return for constitutional reform (such as an elected House of Lords and the introduction of PR for local government). In contrast to Clegg, who is clearly open to a deal, both men have recently suggested that they may not reverse their opposition to a vote. Here lies another reason for Cameron's anxiety: if he cannot deliver an EU referendum, he has vowed, he will resign as prime minister.