NEW YORK (Reuters) - New York City Mayor Bill de Blasio, who emerged as a progressive standard-bearer in 2013 but has struggled amid middling approval ratings to build a national profile, will enter the 2020 presidential race on Thursday, according to a source close to the mayor.

A liberal Democrat, the 57-year-old de Blasio is serving his second four-year term as mayor of the country’s biggest city and will be prevented by term limits from running again in 2021. He is scheduled to appear on ABC’s “Good Morning America” on Thursday after launching his campaign with a video, said the source, who requested anonymity to discuss the rollout ahead of time.

De Blasio’s entry will swell the ranks of Democratic hopefuls to two dozen, all aiming to take on U.S. President Donald Trump, who is expected to be the Republican nominee next year.

Even after six years overseeing a city of more than 8 million people, de Blasio faces an uphill battle to stand out among the Democratic contenders, who include former U.S. Vice President Joe Biden and liberal icons like U.S. Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts.

During his first run for mayor, de Blasio campaigned on reducing income inequality while providing more services for working-class families, themes that have become mainstays for national Democratic figures.

His signature policy accomplishment was establishing universal pre-kindergarten in the largest school system in the country. De Blasio’s administration has also implemented police reforms, paid sick leave, increases in the minimum wage and new identification cards that allow illegal immigrants to access city services.

But the mayor has grappled with setbacks. A federal criminal investigation did not result in charges against de Blasio but nevertheless found the mayor or his associates accepted contributions from donors seeking official favors and then made inquiries to city agencies on their behalf.

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The city is also confronting a persistent housing crisis, including a growing homeless population, despite de Blasio’s push to finance tens of thousands of affordable housing units.

De Blasio’s constituents have not appeared excited about the prospect of their mayor running for president. A Quinnipiac University poll in April found that more than three-quarters of New Yorkers said he should not make a White House bid.

De Blasio has not shied away from criticizing Trump on issues like climate change, immigration and policing.

Earlier this week, the mayor held a news conference inside Trump Tower in Manhattan, where he touted new legislation requiring high-rise buildings to cut emissions and threatened that the Trump Organization would face fines if it does not ameliorate its properties.

Trump’s two oldest sons, Eric and Donald Jr., who run the family company, traded barbs with de Blasio on Twitter, calling the event a political stunt. The president retweeted a message from Eric Trump slamming de Blasio’s stewardship of the Trumps’ hometown.

De Blasio has teased a presidential run for months, holding fundraisers and traveling to key early voting states like New Hampshire and Iowa to meet with activists and voters.

He will hold several events in Iowa on Friday before traveling to another early voting state, South Carolina, for the weekend.