Julian Castro, former U.S. Department of Housing and Urban Development (HUD) Secretary and San Antonio Mayor, announces his candidacy for president in 2020 at Plaza Guadalupe on January 12, 2019 in San Antonio, Texas.

"But with only a month until the Iowa caucuses and given the circumstances of this campaign season, I've determined that it simply isn't our time," he added. "So today it's with a heavy heart and with profound gratitude that I will suspend my campaign for president."

"I'm so proud of the campaign we've run together. We've shaped the conversation on so many important issues in this race, stood up for the most vulnerable people, and given a voice to those who are often forgotten," Castro said in his video announcement.

Former Housing and Urban Development Secretary Julian Castro dropped out of the 2020 Democratic presidential primary on Thursday, ending a progressive and pugnacious bid that emphasized immigration and social issues but rarely made progress in the polls.

A former mayor of San Antonio and HUD secretary during President Barack Obama's administration, Castro campaigned on reforming immigration policy and the criminal justice system. He supported "Medicare for All" and climate change initiatives such as the Paris Accord and Green New Deal.

But his progressive campaign never gained traction in the polls. The RealClearPolitics polling average showed Castro consistently mired at the bottom of the field with less than 2% support.

His fundraising operation also lagged behind his competitors. He reportedly raised just $3.5 million in the third quarter of 2019 — a fraction of hauls raised by pack-leaders Joe Biden, Bernie Sanders, Elizabeth Warren and Mayor Pete Buttigieg.

Castro, 45, was criticized after a September debate, when he attacked Biden by suggesting the then-76-year-old former vice president may have had a lapse in memory.

"Are you forgetting what you said two minutes ago?" Castro asked Biden. The quip drew hoots of displeasure from the debate audience, and critics saw it as a disrespectful attack on Biden's age and mental sharpness.

Biden appeared to let bygones be bygones Thursday morning, complimenting Castro in a tweet.

"It was a privilege to work with @JulianCastro during the Obama Administration, and a true honor to be in this talented field of candidates with him. He led his historic campaign with grace and heart and used his platform to lift the voices of others. Thank you, Mr. Secretary," Biden tweeted.

Castro, whose twin brother, Joaquin, is a Texas congressman, often focused on his history as a child of Mexican immigrants and presented himself as the candidate who best represented the Hispanic community.

But his was not a single-issue campaign. He had proposed increasing funding for the health care, education and housing of indigenous peoples, along with establishing a White House Council on Indigenous Communities.

He also pushed an animal welfare plan that would make animal abuse a federal crime, end the euthanasia of healthy animals in shelters and improve standards for animal welfare in farming practices.

Castro was one of the youngest Democrats vying to defeat President Donald Trump in the 2020 contest. He is the first candidate to quit the race since early December, when Sen. Kamala Harris of California folded her campaign.

Word of Castro's plans to withdraw was first reported by The New York Times.

Some of the remaining candidates shared parting words for Castro on social media.

"America is better because @JulianCastro has dedicated his career to lifting communities and expanding opportunities for all of us," Buttigieg tweeted.

"Congrats Julián on having run a tremendous race that elevated the right issues," wrote entrepreneur Andrew Yang, whose own campaign saw a huge fundraising boost at the end of 2019.

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Warren and New Jersey Sen. Cory Booker also tweeted about Castro.

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