David Cameron will turn to the Thatcherite phrasebook on Monday as his emboldened Conservative party enters the final stretch of a campaign to clinch its first by-election gain from Labour in three decades.

The Crewe and Nantwich by-election on Thursday sets up one of Gordon Brown’s most testing weeks as prime minister, with polls predicting his party will slump to a defeat that will deal a further blow to his authority. Cabinet ministers on Sunday were damping expectations for the outcome of a campaign that Harriet Harman, deputy leader, admitted was “not the most positive”. Amid signs that the chancellor’s unexpected £2.7bn ($5.3bn) give-away to voters may have backfired, Mr Cameron is set to take the offensive on tax and spending with a call to “start living within our means”.