Voters should use the local elections to "punish" Labour for scrapping the 10p tax band at a time of rising living costs, David Cameron has said.
The Tory leader, on a campaign visit to south Wales, said Prime Minister Gordon Brown had let down low-paid workers - despite promises of compensation.
Mr Brown, also in Wales, defended the changes, saying most were better off.
It comes as a poll suggests the Tories have an 18 point lead over Labour - enough to win a general election.
The YouGov poll for the Daily Telegraph put the Tories on 44% to Labour's 26%.
A Commons defeat loomed for Mr Brown on the 10p tax band until Wednesday when the government accepted demands to assist some of those affected by new income tax rates.
Compensation
Chancellor Alistair Darling is proposing to help the two main groups to be hit - pensioners under-65 and younger workers on modest incomes with no children.
"What's important in a sense is not to become bound up in the high decibel exchanges of Westminster but to get out there on to the streets"
Labour rebel Frank Field withdrew an amendment to the Finance Bill enacting this year's Budget, which would have triggered a crisis for Mr Brown's premiership if enough MPs had backed it.
But the Tories are hoping to keep pressure up on the issue and have tabled an amendment of their own, which will force a debate and a vote on compensation measures for those who have lost out on Monday.
Mr Cameron, who was canvassing voters in Cardiff ahead of Thursday's local elections, said: "I've just been listening to candidates and councillors from the Vale of Glamorgan and they have been saying to me that the 10p tax rate is coming up again and again.
"Labour are punishing the low-paid and the people of Wales should turn around and punish the Labour Party."
Economic growth
Mr Brown, also on the campaign trail in south Wales, hailed the government's package of compensation for pensioners and the low-paid who lost out from the abolition of the 10p rate.
But he said the changes to the system that originally triggered the row - announced in his final Budget as chancellor last year - had benefited the "vast majority" of people in Britain.
And he underlined the government's record on employment and economic growth.
"Labour has created in Wales and the UK more jobs than any other government," he said.
Claims Mr Brown's position could be under threat after the 10p tax U-turn have meanwhile been dismissed as "Westminster hysteria" by Olympics Minister Tessa Jowell.
'Difficult time'
In an interview with GMTV, to be broadcast on Sunday, Ms Jowell said Mr Brown remained "up to the job" of prime minister.
She acknowledged it was "a very difficult time for the government".
But she added: "What's important, in a sense, is not to become bound up in the high-decibel exchanges of Westminster but to get out there on to the streets, on to the doorsteps, talk to real people, and for the business, the priorities and the process of government to be informed by the good sense of the British people."
The prime minister's official spokesman earlier described a newspaper story predicting a cabinet reshuffle in an attempt to relaunch the government as "total garbage".
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