Former Labour leader Ed Miliband said that the "nasty party" was "well and truly back" as he took a swipe at Tory plans for older people.
Senior Conservatives also broke ranks to attack their own party's policy on social care costs south of the Border.
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A separate row also erupted after a cabinet minister appeared to downgrade a promise to cut immigration to the tens of thousands to merely an "ambition".
Mrs May herself popularised the term "nasty party" when she warned that Tory faithful that that was how they were seen at their 2002 annual conference.
In the years after the party tried to throw off the nickname, most infamously with David Cameron's 'hug a hoodie' speech.
But Mr Miliband said the Conservative "facade" had been "shattered" as Labour attacked the Prime Minister's decision to scrap the pensions ‘triple lock’ in favour of a less generous ’double lock’.
Labour said that if the ‘double lock’ had been in place for the past seven years, state pensioners across the UK would now be £330 worse off.
Under the plans, Labour said, in one year payments would have risen by a measly £1.36 a week.
The current 'triple lock' ensures the state pension increases in line with wages, inflation or by 2.5 per cent - whichever is highest.
But there have been mounting calls to scrap it, amid warnings of rising inflation caused by last year's Brexit vote and concerns over cost.
The Conservative manifesto promises to keep the 'triple lock' until 2020, when it would be replaced by the 'double lock', of either earnings or inflation.
Labour also hit out at the decision to means test the winter fuel allowance, which will not apply in Scotland.
Shadow chancellor John McDonnell said: "This is a savage attack on vulnerable pensioners, particularly those who are just about managing.
"We will not stand by and allow so many of them to be back in a situation where they have to choose whether they heat or eat."
He predicted that the Tory policies targetting older people, a group much more likely to vote than young people, meant Labour would "win" the keys to Downing Street.
But his prediction came amid reports that Labour insiders believe that the party risks losing almost 90 seats in June.
Meanwhile, the decision to scrap a planned cap on social care costs in England came under attack from a senior Conservative candidate.
Dr Sarah Wollaston, who chaired the House of Commons Health Select Committee during the last parliament, said that older people would face a "miserable lottery" when it came to funding their future care.
Her party insisted that their long-term plan for elderly care would “make the system fairer, better-funded and more sustainable”.
Overnight the Conservative Defence Secretary Sir Michael Fallon told the BBC's Newsnight it was an "ambition" to get immigration numbers down, adding that it was impossible to calculate how much the policy would cost because there was no fixed deadline.
Within hours he was slapped down by Brexit Secretary David Davis who said that the target was a "major imperative" for the Tories.
But Ukip's immigration spokesman John Bickley accused the party of "taking the British people for mugs".
Liberal Democrat leader Tim Farron said that the row was "a reminder Theresa May has set immigration targets throughout her time as Home Secretary and now as Prime Minister and has failed to meet them".
The pledge has now appeared in three successive Conservative general election manifestos.
But in her six years as Home Secretary Mrs May failed to get numbers below 100,000.