Polls have closed in one of the most closely-watched by-elections in decades - with the decision of the voters of Eastleigh having potentially significant implications for all parties.
The Liberal Democrats remain narrow favourites to retain the seat vacated by the disgraced ex-Cabinet minister Chris Huhne when he admitted dodging a speeding penalty.
But a late surge by the UK Independence Party has added spice to what had initially been considered a straight Lib Dem/Conservative contest - the first such battle between the Westminster coalition partners.
Failing to win in the Hampshire constituency - one of the target seats considered crucial to David Cameron securing an overall majority at the next general election - would be a serious blow to the Prime Minister.
And he has been warned by senior backbench MP David Davis, whom he beat to the party leadership, that being pushed into third by Ukip would represent a "crisis".
Ukip leader Nigel Farage, who considered running himself, said he would be "very disappointed" if his party, which has performed well in recent by-elections, came third.
A successful defence of the 3,864 majority secured by Mr Huhne in 2010 would be a conspicuous triumph for the Lib Dems, whose national opinion poll ratings have plummeted since going into government with the Tories.
It has a strong local base, holding all of the local council seats within the constituency, but has seen its campaign overshadowed by sexual harassment claims - strongly denied - against ex-chief executive Lord Rennard.
Defeat would pile further pressure on the leadership of Deputy Prime Minister Nick Clegg who has faced a maelstrom of criticism over his handling of the affair.
Labour sources concede that they have been "badly squeezed" in the by-election and now expect to come fourth in a contest Opposition leader Ed Miliband had hailed as a test for his "One Nation" message.