Jerry Nadler, the House Democratic chair, appeared to almost collapse Friday when he slumped and said he was unwell as he sat beside New York mayor Bill de Blasio at a press conference in the city.

The 71-year-old, a frequent target for Donald Trump, was put on a stretcher and taken to a waiting ambulance for treatment for 'dehydration.'

His spokesperson told NBC4 News he was 'responsive and receiving a check-up.' He was taken to the hospital for medical attention and later tweeted that he was 'feeling much better.'

The drama unfolded at a school in the Upper West Side of Manhattan where he was joining de Blasio - a 2020 candidate - to speak about road safety.

While the mayor was speaking, Nadler slumped in his chair and put his hand to his head. He appeared pale.

De Blasio turned round and offered him what the New York Times reported was Gatorade, saying: 'Jerry, you want some? Jerry?'

Three medical staff in white coats who were attending the event came to give him attention. A voice heard on loudspeaker said: 'Code blue in the gymnasium.'

Collapse: Jerry Nadler slumped in his chair and appeared pale before being offered Gatorade by the mayor. Officials called an ambulance to the New York public school where he fell ill

Aid: New York mayor Bill de Blasio gavee the 71-year-old Nadler water; he was also given an orange

Intervention: Three medical staff who had come to the event in white coats rushed to Nadler's aid

Call off the event: Officials emptied the school hall as concern mounted for Nadler

He was given water and also an orange, and appeared to recover slightly.

After a few minutes, officials closed the hall and ordered the audience to leave, and an ambulance arrived.

Nadler was put on a stretcher and taken to the ambulance, which was backed up in the alley behind the school - P.S. 199 on West 70th Street.

It is not known what hospital the 12-term Congressman was treated at.

Much of the collapse was seen on a YouTube video feed provided by the New York mayor's office but early Friday afternoon the video was taken down.

De Blasio, a Democrat who recently became the 24th Democrat to run for president, was sitting next to Nadler and answering questions when Nadler slumped over.

'I put my hand on his shoulder and I said, 'Jerry, are you OK?' and he didn't respond right away,' de Blasio said afterward, adding that he 'kind of shook' the congressman.

'Medical personnel immediately jumped in,' said de Blasio. 'What was striking was that in just a few minutes you could see him just fully come back to his energetic self. He started making jokes.'

Dr. Ted Long, a medical doctor and a top official with New York City Health and Hospitals, was at the speed cameras news conference and went to Nadler's aid within seconds. He said the congressman was able to answer medical workers' questions right away.

The collapse came after a bruising week for Nadler in which he had been singled out by Trump at a free-wheeling press conference in the White House and been part of efforts to stop rank-and-file Democrats pushing for the president's impeachment.

He also held a session of the committee on Tuesday where former White House counsel Don McGahn declined to attend.

Before: Jerry Nadler tweeted a picture of him arriving at P.S. 199 on the Upper West Side of Manhattan with New York mayor Bill de Blasio

At work: Nadler had spent much of the week in Washington D.C. where he had chaired the House Judiciary Committee session which former White House counsel Don McGahn refused to attend

Nadler is also trying to get Robert Mueller to testify in front of his committee and said on Thursday night that he believed the special counsel wanted to do so in private.

He told MSNBC's Rachel Maddow that Mueller wants to talk to Congress but speculated that he 'doesn't want to participate in anything that he might regard as a political spectacle.'

'He wants to testify in private. I don't know why,' he said.

Nadler also said Mueller is 'willing to make an opening statement' publicly before the hearing moves behind closed doors.

A publicly released transcript, he said, would be required.

'We'd see a transcript. ... We're saying we think it's important for the American people to hear from him and to hear his answers to questions about the report,' the New York Democrat told Maddow.

That prompted Trump to denounce House Democrats on Friday for pursuing Mueller's testimony in their investigation targeting the president.

'I don't know why the Radical Left Democrats want Bob Mueller to testify when he just issued a 40 Million Dollar Report that states, loud & clear & for all to hear, No Collusion and No Obstruction (how do you Obstruct a NO crime?) Dems are just looking for trouble and a Do-Over!' Trump tweeted.

The Fox News Channel, his favorite morning outlet, had aired a short item about House Judiciary Committee Jerrold Nadler's statement Thursday night that Mueller wants to testify behind closed doors.

Trump tweeted that 'Dems are just looking for trouble and a Do-Over!'

Nadler has gone to war with Trump over a number of issues including the administration's refusal to let AG Barr and former White House Counsel Don McGahn testify.

If House Speaker Nancy Pelosi were to give a green-light to impeachment proceedings against the president, the process would begin in Nadler's committee.

But that has put Nadler at the center not just of Trump's rage but of the growing split inside the Democratic caucus over whether or not to impeach the president.

On Wednesday morning House Speaker Nancy Pelosi urged her caucus to unite behind her refusal to impeach Trump and said the work of Nadler and five other committee chairs investigating the president and his finances had to come first.

She used Nadler to address the caucus to convince them of her case, with the Judiciary Committee chair discussing his panel's vote to hold Barr in contempt for not handing over the full, unredacted Mueller report.

He pushed for the full House to vote on contempt of Congress for Barr this month.

He also said Mueller must testify in public, according to a Democratic source in the room. The Judiciary panel and the Justice Department are negotiating on Mueller's testimony and reports indicate the special counsel would prefer to speak behind closed doors.

On Thursday, in the aftermath of Trump storming out of an infrastructure meeting with Pelosi and Chuck Schumer at the White House on Wednesday, the president again attacked Nadler, saying he had 'beaten' him in the past and would do so again.

The two have a long history which goes back to Trump's attempts to develop former railroad yards on the Upper West Side of Manhattan in the early and mid-1990s.

Nadler, then a state assembly member and later a Congressman, opposed the plan - which included the world's tallest tower, which would have Trump's own apartment at the top - and wanted more affordable housing instead.

The outcome was effectively a draw, with Trump developing a complex of apartment blocks but not his tower; many of them have voted to remove the Trump name from their buildings.

At the time Trump lampooned Nadler as 'fat Jerry.' In 2002 and 2003 Nadler had stomach surgery and lost 100lbs. Trump later claimed he was concerned about his rival's health.