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The political arm of the Congressional Black Caucus endorsed Hillary Clinton on Thursday in what has become a tight race for the Democratic presidential nomination, providing an institutional boost as she battles with Senator Bernie Sanders for the support of black voters.

The endorsement comes after the board of the caucus’s political action committee voted to back her, with Mr. Sanders failing to get any votes and two people abstaining.

Nearly half of the Congressional Black Caucus’s 46 members gathered at the Democratic National Committee headquarters Thursday in support of Mrs. Clinton, saying she had spent her career proving her commitment to causes important to them, including poverty and voting rights. They said the endorsement of the board — which is made up of a handful of members of Congress as well as consultants and other prominent black figures — reflects that.

“You judge a person by their results, and there is no question that the person that has obtained the most results and benefits for communities of color and everyone in America, in my opinion — especially getting Democrats elected — it’s not even close. It’s not even close,” said Representative Gregory Meeks, Democrat of New York and the PAC’s chairman. “It’s Hillary Clinton.”

Though he pointed out that more than 33 members had endorsed Mrs. Clinton, Mr. Meeks was careful to say that this did not constitute an endorsement on behalf of the caucus as a whole, which has Republican members as well. In fact, Representative Keith Ellison, Democrat of Minnesota and a member of the caucus, said on Twitter that the board did not consult the other members. (Mr. Ellison has endorsed Mr. Sanders.)

Cong'l Black Caucus (CBC) has NOT endorsed in presidential. Separate CBCPAC endorsed withOUT input from CBC membership, including me. — Rep. Keith Ellison (@keithellison) February 11, 2016

The endorsement comes as Mr. Sanders has been gradually picking up some high-profile black supporters, like the writer Ta-Nehisi Coates and, on Thursday, the singer Harry Belafonte.

Mr. Meeks said that Representative James E. Clyburn of South Carolina — the House’s No. 3 Democrat, who was not present Thursday — would make his own decision. Mr. Clyburn has said he will not endorse a candidate before his state’s Feb. 27 Democratic primary.

Some of those assembled — a group that included Representatives John Lewis, a Georgia Democrat and civil rights leader, and G. K. Butterfield, a North Carolina Democrat and chairman of the caucus — also offered a notably harsh assessment of Mr. Sanders, slamming him in particular for doing little to curb gun violence.

Representative Cedric Richmond, Democrat of Louisiana, younger voters, who provide a significant share of Mr. Sanders’s support, should carefully scrutinize his proposals “because most of the time if it’s too good to be true, it’s too good to be true.”

“When you start saying free college, free health care, the only thing you’re leaving out is a free car and a free loan,” he said. “But who’s going to pay for it? How are you going to pay for it?”

Mr. Meeks said the group would provide the maximum contribution allowable — $5,000 — to Mrs. Clinton’s campaign. Members also said many of them would be traveling to South Carolina to campaign on her behalf, with some lawmakers going this weekend ahead of the state’s crucial primary a week later.

And as some criticized the committee’s appearance at the Democratic Party’s headquarters, Mr. Meeks dismissed the notion that the group’s backing signified that the Democratic political establishment was again rallying to Mrs. Clinton’s side against an outsider.

“We are not the establishment,” he said, as other members laughed. “We are from the streets. Mess with us, and you’ll see we are from the streets.”