New York Mayor Bill de Blasio has said he will seek the 2020 Democratic presidential nomination, making him the 23rd prospective challenger to Donald Trump.

"Donald trump must be stopped. I am Bill de Blasio and I am running for president because it's time we put the working people first," he said in a video announcing his candidacy.

The mayor, who is barred from seeking a third four-year term in 2021, emerged in 2013 as a leading voice for the burgeoning left wing that has reshaped his party.

But he has struggled in the intervening years to build a national profile, and his approval ratings at home have waned after several political setbacks.

Mr De Blasio is scheduled to travel to the early-voting states of Iowa and South Carolina this weekend to begin campaigning.

The mayor plans to emphasise his record of progressive accomplishments, including universal pre-kindergarten, a $15 minimum wage and paid sick leave - all in a city that has a bigger population, more than eight million, than most US states.

But Mr de Blasio faces an uphill battle to stand out among nearly two dozen Democratic contenders, who include former US Vice President Joe Biden and liberal icons like US Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts.

Most New Yorkers appear unenthused about Mr de Blasio's presidential aspirations.

A Quinnipiac University poll in April found more than three-quarters of New Yorkers did not feel he should make a White House bid.

His popularity took a hit after a federal investigation found the mayor made inquiries to city agencies on behalf of donors, though it cleared him of criminal wrongdoing.

Mr de Blasio has denied any misconduct, saying he acted appropriately at all times.

He has sharply criticised Mr Trump on issues like climate change, immigration and policing.

On Monday, he held a news conference inside Trump Tower to call on the Trump Organization to meet newly enacted emissions standards in their skyscrapers, or face significant fines.

In a preview of what could be to come, the event drew scathing insults on Twitter from Mr Trump's two oldest sons, Eric and Donald Jr., who run the family company.

The president retweeted a message from Eric Trump slamming Mr de Blasio's stewardship of his family's hometown.