George Osborne has sought to exploit Labour divisions over welfare cuts - urging "progressive" MPs to unite behind his squeeze on tax credits in a Commons vote today.
The Chancellor said three of the four candidates to lead the Opposition were committed to "an unaffordable welfare state" after they rejected reforms announced in the Budget.
Acting Labour leader Harriet Harman backed moves to reduce the household welfare cap and restrict tax credit and universal credit payments to two children, arguing that there was a clear public appetite.
Warning that the party's general election defeat showed it could no longer afford to oppose all such austerity measures, she announced its MPs would not oppose the Welfare Reform and Work Bill.
But amid a storm of protest from many MPs including would-be leaders Andy Burnham, Yvette Cooper and Jeremy Corbyn, she was forced to accept demands to table an amendment seeking to block it.
Only Blairite candidate Liz Kendall backed Ms Harman's position, later going on to say that she would be prepared to scrap the cuts only if a way could be found to pay for them from other spending.
Writing in The Guardian ahead of the legislation's Second Reading debate, Mr Osborne hailed the package, which also includes a significant increase in the minimum wage, as a "fair deal".
"I believe this settlement represents the new centre of British politics and appeal to progressive MPs on all sides to support us," he wrote.
"Anyone who cares about well-funded public services such as the NHS and schools knows we have to control the costs of a welfare system that has become unsustainable and risks crowding out other areas of government spending.
"Three in four people - and a majority of Labour voters - think that Britain spends too much on welfare.
He added: "I thought British politics had taken a step forward when Labour’s acting leader, Harriet Harman, indicated that she would support at least some of our reforms.
"She accepted the need for a lower benefit cap and the limit on the number of children eligible for tax credits, arguing that Labour cannot continue to ignore voters’ views on welfare.
"She recognised something else important in a democracy: that oppositions advance only when they stop blaming the public for their defeat and recognise that some of the arguments made by political opponents should be listened to – just as a previous Conservative opposition realised 15 years ago when it accepted the case for a minimum wage.
"Depressingly, the Labour leader has been forced to retreat from her sensible position after Len McCluskey accused her of 'running up the white flag' and the leadership candidates Andy Burnham and Yvette Cooper joined Jeremy Corbyn in undermining her.
"With the vote coming tonight, I urge moderate Labour MPs not to make the same mistake as in the last parliament, when they refused to support each and every welfare reform we proposed. I say: vote with us today.
"Lower welfare in return for a national living wage is widely recognised as a fair deal. It has been proposed on the left as well as the right.
"Welfare reform is not just about saving money. It’s about transforming lives, and social justice. This is the new centre of our politics. All those who call themselves progressives should join us."