Peter Mandelson will today urge Labour activists to “hold our nerve” in the face of growing confidence in Conservative ranks that they have landed a crucial blow over national insurance rises.
With Gordon Brown expected to call the General Election tomorrow, surveys suggest that Tory support has started firming up.
The Conservatives are convinced that their promise to reverse planned increases in national insurance contributions for most workers has boosted their prospects.
The Tories intend to follow the voter-friendly announcement within days by setting out plans for new tax breaks for married couples.
Labour will retaliate by claiming the Tories will be forced to put up value added tax or implement swingeing cuts to pay for their pledge.
Lord Mandelson, who will direct Labour's election campaign, says in his message: “Our task is to hold our nerve and stick to our guns.”
The parties will launch their manifestoes next week, with the first TV debate between the main party leaders taking place on April 15.