A riled up Bernie Sanders served notice yesterday that he will not be turning any cheeks as his battle for the Democratic presidential nomination with Hillary Clinton enters a new, red-hot phase, declaring: “I’m not going to get beaten up, I’m not getting lied about. We will fight back.”

Under intense pressure to sustain momentum after a recent string of victories including in Wisconsin this week, Mr Sanders bared his claws after the Clinton camp said on Wednesday that it would seek to “disqualify’ him in the eyes of voters in New York, the next big state to vote.

Mr Sanders took that to mean that his credentials to serve in the White House were being brought into question and hit right back suggesting it was Ms Clinton’s qualifications that should be examined, citing her record of voting for the Iraq war and taking money from Wall Street.

“Are you qualified to be president of the United States when you're raising millions of dollars from Wall Street, an entity whose greed, recklessness and illegal behavior helped destroy our economy?” the Senator from Vermont asked yesterday at a news conference in Philadelphia.

In pictures: US Elections 2016 Show all 15 1 /15 In pictures: US Elections 2016 In pictures: US Elections 2016 Democratic U.S. presidential candidate Bernie Sanders speaks to supporters after rival candidate Hillary Clinton was projected as the winner in the Nevada Democratic caucuses Reuters In pictures: US Elections 2016 Democratic presidential candidate Hillary Clinton takes photos with workers at her campaign office in Des Moines, Iowa AP In pictures: US Elections 2016 Republican presidential candidate Mike Huckabee, second from left, prays before lunch with supporters at Drake Diner in Des Moines, Iowa Reuters In pictures: US Elections 2016 Democratic presidential candidate and former Maryland Governor. Martin O'Malley, speaks during a campaign stop in Waterloo, Iowa AP In pictures: US Elections 2016 Democratic presidential candidate Bernie Sanders speaks, as his wife Jane O’Meara Sanders looks on, at a campaign event at Iowa State University Getty In pictures: US Elections 2016 Republican presidential candidate Marco Rubio speaks at St. Ambrose University in Davenport, Iowa Reuters In pictures: US Elections 2016 Republican presidential candidate Ben Carson speaks at a campaign event at Fireside Pub and Steak House in Manchester, Iowa. Getty In pictures: US Elections 2016 Republican presidential candidate Rick Santorum visiting supporters at a house party in West Des Moines, Iowa Reuters In pictures: US Elections 2016 Republican presidential candidate Donald Trump at a campaign event in Cedar Rapids, Iowa AP In pictures: US Elections 2016 Republican candidate Ted Cruz campaigns at Greene County Community Centre in Jefferson, Iowa AP In pictures: US Elections 2016 Senator Rand Paul speaks during a Caucus rally at his Des Moines headquarters in Iowa Getty In pictures: US Elections 2016 Republican candidate Jeb Bush speaks at a campaign event in Des Moines, Iowa AFP In pictures: US Elections 2016 Former Alaska Governor Sarah Palin introducing the arrival of Donald Trump during a campaign rally in Cedar Rapids, Iowa Reuters In pictures: US Elections 2016 A portrait of Democratic candidate Bernie Sanders at his campaign headquarters in Des Moines, Iowa Getty In pictures: US Elections 2016 Campaign badges on sale ahead of a Trump rally at the Ramada Waterloo Hotel and Convention Centre in Waterloo, Iowa Getty

For her part, Ms Clinton stood among a throng of reporters outside Yankee Stadium in New York and attempted to cool the campaign coals. “I will take Bernie Sanders over Donald Trump or Ted Cruz any time, so let's keep our eye over what's at stake in this election,” she said.

The former first lady, who represented New York in the US Senate for eight years and cannot afford to lose it to Mr Sanders when it votes on 19 April, then plunged into a nearby subway station to take a train south to Manhattan, a stunt clearly aimed at reminding voters of a recent Sanders gaffe when he said tokens are used to pay for rides, which hasn’t been true for years.

The tone-change in Mr Sanders, who calls himself a democratic socialist, may stem from his circle of aides who have long worried that he has been insufficiently tough with Ms Clinton. He has still, for instance, to make any attempt to use the ongoing FBI investigation into her use of a private email server while Secretary of State as an issue against her on the campaign trail.

“This is not the type of politics that I wanna get in,” Mr Sanders conceded to reporters. But he swiftly shed all reluctance, claiming that Ms Clinton had started it all by coming after him “in some kind of really, uncalled-for way.” He added: “If Secretary Clinton thinks that I just come from a small state of Vermont and we're not used to this, well, we will get used to it fast.”

The candidate also took a beating meanwhile from Jeff Immelt, the CEO of General Electric, one of America’s largest and most recognised companies, who lambasted him in an article in the Washington Post for allegedly linking his company with so-called “corporate greed”.

“It's easy to make hollow campaign promises and take cheap shots in speeches and during editorial board sessions, but US companies have to deliver for their employees, customers and shareholders every day,“ Mr Immelt wrote, before adding sardonically that his 124-year-old company “has never been a big hit with socialists”.