Prime Minister David Cameron is expected to use the launch of his party's manifesto on Tuesday to promote a ‘Conservative dream’ aimed at seducing low-paid workers as he seeks to make a shift towards more positive campaigning.

With just over three weeks before what is shaping up to be Britain's closest election since the 1970s, Cameron and his Conservatives are under pressure to put clear water between themselves and the opposition Labour Party in opinion polls, most of which have the two neck-and-neck.

One poll conducted on the eve of the manifesto launch gave the Conservatives a six-point lead over Labour, boosting party morale. But the overall trend still points to a dead heat.

At stake is more than simply who will govern the $2.8 trillion economy: Cameron has promised a referendum on European Union membership while Scottish nationalists, who want Scotland's independence, are seeking a kingmaker role.

Some Cameron allies fret that the central economic message of his campaign - that the centre-right party need another term to complete the economic recovery it has nurtured - has been too abstract and relied too heavily on dry economic data.

On Tuesday, Cameron will try to broaden that message, and, in the spirit of the late Margaret Thatcher, is expected to pledge expanded help for people to buy their own homes and to ensure that more low-paid workers no longer have to pay taxes.

‘At the heart of this manifesto is a simple proposition,’ Cameron is expected to say, according to advance extracts provided to some local media. ‘We are the party of working people, offering you security at every stage of your life.’

His speech will be targeted at winning over working class voters who have switched their allegiance to the anti-EU UK Independence Party (UKIP) and wavering Labour supporters.

Cameron's campaign narrative casts his party as the defenders of a strong economic recovery pitched against the ‘chaos and incompetence’ of Labour who left Britain in 2010 with its biggest peacetime deficit since World War Two.

Party strategists hoped that the economic argument coupled with what they saw as the unusually weak leadership qualities of Labour chief Ed Miliband would have put the Conservatives in a commanding opinion poll lead by now.

Instead, the Conservatives find themselves under mounting pressure to do or say something different to break the poll deadlock.