When I recently interviewed Conservative MP Robert Halfon, who first called for the reintroduction of the 10p tax rate, he lamented how Labour's "brilliant" campaign against the abolition of the 50p rate had defined the Tories as "a party only interested in cutting taxes for millionaires". More than any other single measure, the move retoxified the Conservative brand and confirmed the Tories' status as "the party of rich". Every time that David Cameron defends an unpopular tax rise or spending cut, Ed Miliband is able to remind voters that he has simultaneously chosen to reduce taxes by an average of £107,500 for 8,000 income-millionaires.  

With just over a month to go until the tax cut is introduced on 6 April, Labour is stepping up its campaign against the measure. The party has today launched a new ad featuring Cameron writing a cheque for £100,000 to "a millionaire" and a clock counting down to "Tory Millionaire's Day". In response, expect the coalition to point out that the new 45p rate is, as Danny Alexander recently noted, still higher than the 40p rate seen for 155 of the 156 months that Labour was in power. 

Alongside the new campaign, I'm told that Labour, encouraged by how Barack Obama forced Mitt Romney onto the defensive over his tax bill, will continue to challenge the PM to say whether he will benefit from the reduction in the top rate. Private polling by the party has previously shown that 62 per cent of voters, including 46 per cent of Conservative supporters, believe he should "come clean and tell people honestly whether he is personally benefiting". 

Unlike George Osborne, who said last year that he would not gain from the move, Cameron has so far refused to say whether he will. When challenged on this subject by Stephen Pound MP at PMQs earlier this month, the PM replied evasively that he would "pay his taxes". Under ever-greater pressure from Labour, the Tories will need to decide whether this strategy is sustainable.