Voters go to the polls today in the closely-contested Eastleigh by-election that is poised to heap pressure on David Cameron.
The Hampshire constituency has previously been held by the Conservatives and is one of the seats viewed as key to the party winning an outright majority at the 2015 general election.
But the Liberal Democrats are desperately fighting to hang on to it to give a vital morale boost to the grassroots after support hit the doldrums on entering coalition.
Their campaign has been marred by allegations that the party failed to take proper action to deal with claims that senior peer Lord Rennard sexually harassed party activists.
The UK Independence Party claimed it was taking many votes from each coalition party and leader Nigel Farage said he would be "very disappointed" if his party came third.
Former Tory leadership contender David Davis warned Mr Cameron would face unrest from backbenchers if the party was pushed into third place.
"Let's be clear, it's not going to dislodge David Cameron," he told reporters. "He's going to be there till the next election, but the simple truth is that it will make things more uncomfortable in the House of Commons."
The by-election seeks a replacement for Lib Dem ex-Cabinet minister Chris Huhne, who resigned in disgrace after admitting perverting the course of justice.