Center-left Labour and the center-right Conservatives have dominated British politics for the past 65 years, aided by an electoral system that uses 'first past the post voting' in 650 constituencies to carve up parliament.

But an impressive performance by the leader of the centrist Liberal Democrats in Britain's first-ever pre-election debate last Thursday has turned opinion polls on their head before the May 6 general election.

Labour and Conservatives have since upped their attacks on the Liberal Democrats and their leader Nick Clegg - having largely ignored them previously in the campaign.

Surveys published on Sunday and Monday showed support for the Liberal Democrats had soared to 32 and 33 percent, respectively, narrowly ahead of the Conservatives and up to seven points ahead of Labour.

It is difficult to predict to what extent the percentage share of the overall vote will translate into seats. But some political analysts say a swing to the Liberal Democrats from the two larger parties would hit the Conservatives hardest.

"If things stand where they are at the moment, on the whole, (Labour Prime Minister) Gordon Brown will be much happier than (Conservative leader) David Cameron," said Philip Cowley, Professor of Parliamentary Government at the University of Nottingham.

That is because to secure a victory, the Conservatives need to win 117 seats now held by their opponents, so-called marginal constituencies.
 
TACTICAL VOTING

A poll by the Telegraph newspaper last week found the Conservatives had a 12 point lead over Labour in 100 marginal constituencies.

Tellingly, however, the poll also found that of the 100 constituencies surveyed, the Conservatives would pick up 74 of the 80 Labour seats but none of the 20 seats held by the Liberal Democrats.

That is a serious blow for the Conservatives, who have invested considerable money, much of it donated by wealthy backer Michael Ashcroft, into trying to influence marginal seats.

"In the 100 marginal seats where the election is going to be won and lost a three-way contest is the one thing that can defeat Ashcroft's money and tactics," said Simon Lee, senior politics lecturer at Hull University.

"Tactical voting will become a real issue - that's why the Conservatives are so worried." Tactical voting - where a Labour supporter votes Liberal Democrat to prevent the Conservatives from winning the seat, or vice versa - is commonplace. Such voting intentions do not tend to show up in opinion polls until very close to the election date.

The possibility of an inconclusive election outcome, or hung parliament, has knocked sterling because financial markets reason that a coalition government would find it difficult to implement the spending cuts needed to bring Britain's budget deficit down from above 11 percent of GDP.

But some political analysts say it is by no means clear that a hung parliament would result in paralyzed government, particularly given the shared view of all three parties on the need to tackle Britain's debts.
 
CONSERVATIVE-LIBERAL DEMOCRATS PACT?

For all the mud-slinging of the campaign, a pact between the Conservatives and the Liberal Democrats is possible.

"While most people think it more likely that the Liberal Democrats would be able to reach some post-election deal with Labour than with the Conservatives, their voting over the past decade indicates the opposite," Cowley wrote in an article on voting patterns.

In the 2005-6 parliamentary session the Liberal Democrats sided with the Labour government in just 21 percent of votes where the party leadership dictated how deputies should vote. They were more than three times as likely to side with the Conservatives on votes than with Labour, he noted.

It is more likely that the social liberal Liberal Democrats would, by inclination, prefer to work with a center left Labour-led government than a Conservative party that has a history of social spending cuts.

However, it could prove difficult for Clegg to pursue a Labour deal if Labour ends up with a smaller share of the vote than the Conservatives even if it has the most seats, particularly as the Liberal Democrats have long campaigned for electoral reform that better links national votes to seats in parliament.

"It's perfectly normal in European politics for parties to slag each other off for the weeks of the campaign and then come together at the end to do a deal," said Cowley.