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Bernie Sanders has described Hillary Clinton as unqualified to be president, as the two candidates prepare for an increasingly bitter Democratic primary in New York.

Since Mr Sanders trounced Mrs Clinton in Wisconsin on Tuesday by a double-digit margin, both candidates and their campaigns have become increasingly antagonistic. The Clinton camp has attempted to paint Mr Sanders as out of his depth, while he has attacked Mrs Clinton for her ties to special interest groups. 

In Philadelphia on Thursday, Mr Sanders told a meeting of trade union workers that he was preparing to go after Mrs Clinton more aggressively after she said that an interview with Mr Sanders in the Daily News had “raised a lot of very serious questions” about whether he knew how to execute the policy proposals he had been raising on the campaign trail. 

He said Mrs Clinton’s comments this week had resulted in headlines such as “Clinton questions whether Sanders is qualified to be president”, which ran in the Washington Post. 

“My response is: well, if you want to question my qualifications, let me suggest this. That maybe the American people might question your qualifications Madam Secretary when you voted for the war in Iraq, the most disastrous foreign policy blunder in the history of America.”

At a rally in Pennsylvania on Wednesday, Mr Sanders said: “I don’t believe that she is qualified if she is through her super-PAC taking tens of millions of dollars in special interest money.

“I don’t think you are qualified if you’ve supported virtually every disastrous trade agreement which has cost us millions of decent paying jobs.”

The Democratic race for the White House has seen considerably fewer fireworks than the Republican contest, but that changed over the past week with Mrs Clinton and Mr Sanders turning their attention to one another, rather than focusing on GOP opponents Donald Trump and Ted Cruz.

The shift reflects increasing concern in the Clinton camp that while she is still likely to beat Mr Sanders and win the party’s nomination, she is not performing as well as many of her campaign staff had expected.

Many members of her team had predicted that she would have the nomination wrapped up by the beginning of April, but she is now fighting for states once considered easy wins.

A new poll by Quinnipiac University shows Mr Sanders is shrinking Mrs Clinton’s lead in Pennsylvania, where primaries take place on April 26. Having trailed her by 38 points, he is now just 6 points behind, according to the poll. Mrs Clinton is expected to take 50 per cent of the Democratic vote, and Mr Sanders 44 per cent. 

In New York, Quinnipiac predicts Mrs Clinton — who was campaigning in the state on Thursday — will take 54 per cent of the vote on April 19 versus 42 per cent for Mr Sanders. 

While a loss in the Empire State would not erase her lead in delegate numbers or block her path to the nomination, it would be a significant psychological blow to her campaign. Mrs Clinton spent eight years as a New York senator, and has spent days criss-crossing the state and attending fundraisers there to ensure she shores up her support.

Mr Sanders’ camp has blamed the increasingly heated rhetoric on Mrs Clinton’s camp which is fighting back after a string on losses in states including Wisconsin, Washington, Utah and Idaho. 

“I know that they are getting nervous,” Mr Sanders told the rally of union workers in Philadelphia. “They’ve lost seven out of the last eight caucuses and primaries.”

However, the Vermont senator is under pressure for the Daily News interview in which he struggled to identify how he would break up the banks, a core tenet of his campaign platform, and said he would grant immunity to the gun manufacturer that produced the weapon used in the Sandy Hook shootings. 

In a Washington Post editorial, Jeffrey Immelt, chief executive of General Electric, took aim at Mr Sanders, noting that he had never bothered to visit GE’s aviation plant in Rutland, Vermont, which boasted more than 1,000 employees, and was wrong to paint the company as supposedly “destroying the moral fabric” of America. 

“GE has been in business for 124 years, and we’ve never been a big hit with socialists. We create wealth and jobs, instead of just calling for them in speeches,” Mr Immelt wrote.

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