Labour is targeting Nick Clegg’s parliamentary constituency in a bid to oust the Liberal Democrat leader at the general election.

The “decapitation strategy” focused on Sheffield Hallam comes after a poll showing Labour just three points behind the Liberal Democrats.

While the seat is not on Labour’s official target list, Mr Clegg’s personal unpopularity is seen as making him particularly vulnerable despite his large majority at the 2010 election.

A senior opposition figure told The Times newspaper that Labour saw the seat as worth a “shot from outside the box” in terms of expending resources to win it.

The Liberal Democrat leader has put his own personal stamp on his party’s relationship with the Conservatives and success in ousting him could make it more difficult for the Liberal Democrats to form a new coalition with the Tories after the next election.

Labour’s candidate in the seat, Oliver Coppard, this morning said it was no secret that Labour wanted to win in Sheffield Hallam.

“I wonder if it was the leaflets, website or Facebook that gave it away?” he tweeted.

He also goaded Nick Clegg over a report that he was now spending two days a week in the constituency, an apparent increase on his previous attendance.

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The local party’s Twitter feed paints a highly active picture of Labour’s campaign in the seat, with evidence of a recent visit from former Labour election coordinator Tom Watson and attention from Miliband insider Jon Ashworth.

A poll by Lord Ashcroft conducted last month put Labour’s candidate on 28% in the seat, up from only 16% at the last general election. Nick Clegg is still slightly ahead on 31%, but is down dramatically from the 53% he gained in 2010.

Some of the increased opposition to Nick Clegg has benefited the Green Party rather than Labour, putting the left-wing environmentalists on 9%. If Labour were able to attract these voters it would win in the constituency. The Conservatives are on 19%.

Labour’s election leaflets in the constituency have focused on the closeness of the battle between Labour and the Lib Dems.