Nick Clegg has predicted a "great victory" for the Liberal Democrats in the critical Eastleigh by-election.
Rallying supporters in the Hampshire town, the Deputy Prime Minister admitted the vote was "one of the most exciting and closely contested byelections" he could remember.
He was speaking as voters prepared to go to the polls in an election that pits the two coalition partners against each other, as the Tories were warned that slipping behind UKIP would lead to a "crisis".
Pollsters said the Lib Dems retained a narrow lead, and said a shock win for UKIP "should not be entirely discounted."
On Wednesday evening, Labrokes said the Lib Dems were 1/3 favourites, with the Tories at 4/1, UKIP 6/1 and Labour 100/1 outsiders.
The Lib Dems know they have to hang onto one of their flagship seats, which is up for grabs after the resignation of disgraced former Cabinet minister Chris Huhne.
But they have had to battle potentially damaging allegations about the party's response to sexual harassment complaints against its former chief executive, Lord Rennard.
On Wednesday night it was reported that one of the women had offered to make a complaint against the peer - who denies the allegations - contradicting the official version of events.
Embarking on last-ditch campaigning on Wednesday night, Clegg was forced to deny that the admission that concerns about Lord Rennard's conduct had been a "background" issue at the time of his resignation amounted to another shift in his position.
"I don't think it is actually an elaboration on what I have been saying for days now, which is that there were general concerns back in 2008 and into 2009, and Lord Rennard stood down in the spring of 2009," he said.
Outspoken Maria Hutchings is flying the flag for the Tories
The Conservatives were put under pressure by one of their senior MPs, former leadership candidate David Davis, who said finishing behind UKIP would cause a "crisis" for the Tories.
He said: “I think if we came third it would be a crisis, I think that’s the case, and if it’s a close second with UKIP on our tail it will also be uncomfortable."
Labour's candidate was targeted by David Cameron during Prime Minister's Questions on Wednesday over comments he made about Margaret Thatcher, as his party trailed in fourth place.
Patrick Brione, the director of research for opinion pollsters Survation, said the polls did not tell the whole story with the turnout of each party's core supporters set to be decisive.
With a stronghold on the local council, this is likely to favour the Lib Dems.
But Brione also said Nigel Farage's UKIP were "within touching distance of second place" and added: "The possibility that they could pull off a surprise outside victory, as George Galloway did in Bradford West a year ago, should not be entirely discounted."