Among those who introduced Sanders was Alexandria Ocasio-Cortez, his latest high-profile endorsement.

Basking in loud chants of “Bernie’s back,” Democratic presidential candidate Bernie Sanders has gone back to campaigning weeks after being sidelined by a heart attack, telling supporters he was ready to resume “the epic struggle” for the White House.

Energised by thousands of backers gathered for a rally in Queens, New York City, the 78-year-old Vermont senator said: “When I look at this huge crowd, brothers and sisters, I have no doubt that the political revolution is going to sweep this country, sweep Donald Trump out of office and bring the change that this country has long needed.”

He said he was “more ready than ever to carry on with you the epic struggle that we face today. I am more than ready to assume the office of president of the United States.”

To raucous applause, he declared: “I am back.”

It was the start of what he is calling a “vigorous” return to campaigning as he tries to give his stagnant bid an instant shot of energy.

Among those who introduced Sanders from the stage was Alexandria Ocasio-Cortez, Sanders’ latest high-profile endorsement.

Sanders had emergency heart surgery this month but insists that he is more committed than ever to the 2020 race. With the first voting contests less than four months away, he has some work to do.

Beyond health concerns, Sanders suddenly finds himself looking up at progressive rival Elizabeth Warren and establishment favourite Joe Biden in the polls. Now he must reassure voters that he has the physical stamina to go forward while addressing broader concerns that his policies may be too far left to defeat Trump in a general election.

The endorsement from Ocasio-Cortez, the 30-year-old high-profile progressive, “sends the message that the movement is growing, that it’s gaining influence, that it’s gaining traction,” Sanders’ campaign manager Faiz Shakir said.

While Sanders pledged during this week’s presidential debate to move forward with a “vigorous” campaign, he is moving cautiously in the short term. The rally in Queens was his only scheduled appearance before he returns to Iowa late next week.