The recent surge in the Liberal Democrats' popularity appears to have pre-dated last Thursday's leaders' debate. An ICM poll conducted on Wednesday and Thursday (i.e. presumably mostly before the debate) showed support up to 27% from 20% in the previous survey over 9-11 April.

This pick-up probably reflects a favourable response to Wednesday's manifesto launch and in particular the pledge to raise the personal income tax allowance to £10,000. Tax policies are having a significant influence on voting intentions – the Conservatives had benefited from their opposition to Labour's planned national insurance rise but their lower-tax mantle has been stolen by the Lib Dems.

A Tory fight-back strategy, therefore, must aim to undermine the Lib Dems' claim that most voters will benefit from their tax plans, with the £16.8 billion estimated cost of the higher personal allowance "paid for in full by closing loopholes that unfairly benefit the wealthy and polluters". The largest offset (£5.5 billion) is from extending Labour's restriction of pension tax relief to all higher-rate earners, not just those with gross income including pension contributions of more than £150,000. The swingeing rise in air transport taxes (£3.3 billion) will also be widely felt, as will the new levy on bank profits (£2.2 billion), which will be passed on in borrowing / saving rates. Small business owners and buy-to-let investors, meanwhile, will be hit by the equalisation of capital gains and income tax rates.

Earlier posts suggested that less optimistic voter perceptions of the economy would undermine Labour support as the election approached. There is evidence of this effect – both the EU Commission and Nationwide consumer confidence indicators fell in March – but the polls have been dominated by the swing to the Lib Dems. This, however, looks vulnerable to greater scrutiny of the party's tax plans, as well as a less impressive performance by Mr. Clegg in the remaining debates.