Sen. Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-N.J.) on Thursday began calling members of Congress informing them he is running for president and is quietly making overtures to members for support, three congressional sources told The Hill.

"Yes, he is reaching out to members for their support,” said a former Democratic aide with direct knowledge of Booker’s intentions. "He's going to do it during Black History Month,” which starts on Friday.

“I don't know if it's going to be tomorrow, I just know it's going to be soon."

Among those who received a call Thursday were senior members of the Congressional Black Caucus (CBC), of which Booker is a member.

“He’s making calls,” a fourth source, Rep. Frederica Wilson Frederica Patricia WilsonHarris calls it 'outrageous' Trump downplayed coronavirus House passes bill establishing commission to study racial disparities affecting Black men, boys Florida county official apologizes for social media post invoking Hitler MORE (D-Fla.), confirmed to The Hill on Thursday night. “He left me a voice message. I have to call him back.”

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A Booker spokesman declined to comment for this story Thursday night. But on Friday morning, the first day of Black History Month, Booker posted a video — with images from inner cities and the civil rights movement — formally announcing his presidential bid.

"Over 20 years ago, I moved into the central ward of Newark to fight slumlords and help families stay in their homes," Booker says in the video titled "Rise." "I still live there today, and I'm the only senator who goes home to a low-income, inner-city community — the first community that took a chance on me."

Booker’s entry into the 2020 Democratic presidential primary race was widely expected, but his announcement on the heels of the successful presidential launch of Sen. Kamala Harris Kamala HarrisJoe Biden looks to expand election battleground into Trump country Fox's Napolitano: Supreme Court confirmation hearings will be 'World War III of political battles' Rush Limbaugh encourages Senate to skip hearings for Trump's SCOTUS nominee MORE (D-Calif.) suggests he and his team realized he could not allow his Senate colleague and fellow CBC member build too much momentum, some Democratic observers said.

Both Booker, 49, the former mayor of Newark, and Harris, 54, the former California attorney general, have been vying for endorsements from CBC members — a sign of the importance of the black vote in the Democratic primary, especially in states like South Carolina, Ohio and Georgia.

On Wednesday, both Booker and Harris attended the CBC’s annual policy retreat for more than an hour. Though their 2020 presidential bids were not the focus, both senators worked the room during breaks.

More than a dozen CBC members interviewed by The Hill said they were not ready to endorse anyone yet with the 2020 field continuing to take shape.

As he left the meeting in the Capitol’s basement, Booker sidestepped a question about whether he felt any pressure to quickly launch a presidential bid now that Harris had begun to win congressional endorsements from lawmakers in her home state.

“I’m excited for the candidates already out there. It’s really good for the Democratic Party and it’s just a good thing,” Booker told The Hill. “I’m excited about Kamala’s candidacy. It’s incredible. It’s historic.”

The former Democratic aide said Booker will have a leg up on Harris with CBC members, because he's been a more active member of the group.

"He rarely misses a CBC meeting,” the former aide said. “Kamala Harris — great member — but that's just not her thing. ... So I could easily see Cory Booker getting more of their support."

The aide compared the situation to 2008, when former President Obama thought he'd lock down the support of CBC members because he was a part of the group. "But he never went to a CBC meeting," the former aide said, which allowed Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE to gain the support of much of the caucus.

"Cory Booker has invested in the CBC relationships."

If Harris wins the nomination and defeats President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE, she would become the first woman to win the White House and the second African-American after Obama.

The Democratic primary is expected to be crowded, however. Aside from Harris and Booker, Sens. Elizabeth Warren Elizabeth WarrenJudd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report Biden's fiscal program: What is the likely market impact? MORE (D-Mass.) and Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (D-N.Y.) are already running. So are Rep. Tulsi Gabbard Tulsi GabbardRepublicans call on DOJ to investigate Netflix over 'Cuties' film Hispanic Caucus campaign arm endorses slate of non-Hispanic candidates Gabbard says she 'was not invited to participate in any way' in Democratic convention MORE (D-Hawaii), former Rep. John Delaney John DelaneyCoronavirus Report: The Hill's Steve Clemons interviews Rep. Rodney Davis Eurasia Group founder Ian Bremmer says Trump right on China but wrong on WHO; CDC issues new guidance for large gatherings The Hill's Coronavirus Report: Kansas City Mayor Quinton Lucas says country needs to rethink what 'policing' means; US cases surpass 2 million with no end to pandemic in sight MORE (D-Md.), former Housing and Urban Development Secretary Julián Castro and Pete Buttigieg, mayor of South Bend, Ind.

“The field isn’t even half full yet,” one Democratic senator quipped.

Updated Friday at 8:05 a.m.