Members of the Ward 21 Democratic Party Committee, which represents residents of the Allston, Brighton, and Fenway neighborhoods, voted unanimously Saturday morning to endorse U.S. Rep. Edward J. Markey of Malden in his primary battle against U.S. Rep. Stephen F. Lynch of South Boston.

Markey and Lynch, as well as three Republicans, are seeking the U.S. Senate seat vacated Feb. 1 by recently appointed Secretary of State John F. Kerry.

Saturday’s caucus meeting took place at the Brighton Branch of the Boston Public Library and was attended by just 12 eligible voters. To be eligible, voters must be registered Democrats and residents of the Ward 21 polling district and must attend three meetings of the committee within a year.

State Senator Will N. Brownsberger ’78 represented the Markey campaign, and spoke extensively about Markey’s experience authoring, sponsoring, and supporting climate change legislation in Congress.

Brownsberger, who expressed his intention to run for Markey’s seat should it be vacated, also referenced Markey’s support of abortion rights in Congress in contrast to Lynch’s more socially conservative position on the issue. While Lynch has historically described himself as an antiabortion Democrat and has voted to restrict access to abortion more often than not in Congress, he has said in recent weeks that he does not support the overturning of the Supreme Court’s 1973 decision in Roe vs. Wade.

“You have a candidate who for many years has been a strong advocate of a woman’s right to choose, and a candidate [Lynch] who, until recently, has not,” Brownsberger said.

Brownsberger was not countered by a surrogate from the Lynch campaign. Because no representative from the Lynch campaign attended the meeting, the South Boston congressman was left without a voice before the caucus and without enough support to even qualify for the endorsement ballot. As a result, Markey’s endorsement was decided by an up-or-down vote.

At the meeting, voters voiced excitement about working for Markey, drawing parallels between the campaign ahead and their experiences canvassing for former Harvard Law School professor Elizabeth Warren during her successful U.S. Senate campaign last year. The committee discussed the coordination of voter-registration efforts for the coming campaign, among other things.

In remarks after the endorsement vote, local committee chair Lauren Mattison alluded to the popularly held sentiment that in the heavily blue Commonwealth, whoever wins the Democratic nomination will be heavily favored against any of the three Republican contenders—former U.S. Navy Seal Gabriel E. Gomez, former U.S. Attorney Michael J. Sullivan, and State Rep. Daniel B. Winslow—vying for their party’s nomination.

“It seems like the [Democratic] primary might be the more competitive election,” Mattison said. “Now that we’ve made an endorsement, we can start to do things for Ed Markey.”

But Boston City Councilor At-Large Felix G. Arroyo, who did not vote, urged the group not to color the open Senate seat blue just yet.

“Remember, Scott Brown wasn’t supposed to win, until he did,” said Arroyo, referring to the former Republican congressman who in 2010 beat Mass. Attorney General Martha M. Coakley in a special election following the death of Senator Edward M. Kennedy ‘54-’56.

Party primaries will be held on April 30, and the special election for the senate seat is scheduled for June 25. Kerry’s seat is being filled on an interim basis by William “Mo” Cowan, a lawyer and former chief of staff for Mass. Gov. Deval L. Patrick ’78.

—Staff writer Matthew Q. Clarida can be reached at clarida@college.harvard.edu. Follow him on Twitter @MattClarida.