A bid to grant subpoena power to the watchdog commission that oversees the Los Angeles County Sheriff’s Department will go to voters in March 2020, after failing to gain adoption by the Board of Supervisors.

Backers of the “Reform L.A. Jails Initiative’’ gathered nearly a quarter-million signatures in support of the measure, which would also give the Civilian Oversight Commission a mandate to study reducing the county jail population and reallocating jail construction dollars into alternatives to incarceration.

“This movement is happening,’’ Megan Dobkin told the board. “(I’m) a 45-year-old White suburban mom who drives a minivan … and this movement has reached me.’’

Supervisor Sheila Kuehl said more of the county’s 10 million-plus residents should be allowed to weigh in.

“I understand the hope that went into this initiative,’’ Kuehl said. “I personally think it should go in front of the voters.’’

Members of the coalition behind the ballot initiative include the American Civil Liberties Union of Southern California, Dignity and Power Now, White People 4 Black Lives, Black Community Clergy and Labor Alliance, Black Lives Matter—Los Angeles and more than 140 other organizations, according to its backers.

Kuehl, the only board member to offer comment, said she believes the current arrangement, under which the Office of Inspector General acts as the investigative arm of the commission, provides more oversight.

But advocates for reform say the watchdog agency needs to be able to investigate deputy misconduct by demanding deputy testimony, records and evidence.

“The issue is that this body … lacks the immediate and real authority to do what they were tasked to do,’’ said Esther Lim of the ACLU of Southern California.

In a letter to the board, the county’s lead lawyer argued subpoena power would be ineffective in gaining access to more information, particularly given passage of Senate Bill 1421, which expands public access to records related to police shootings. Subpoenas for non-public information could be successfully blocked, County Counsel Mary Wickham said.

Advocates and community organizers pointed to continuing deputy misconduct, including sexual assault of female inmates, racial profiling and alleged drug trafficking by deputies, in urging the board to adopt the measure as written.

“The Civilian Oversight Commission needs additional authority to ensure transparency and accountability in law enforcement in L.A. County,’’ commission Vice-Chair Priscilla Ocen said in a statement. “It will allow the community to ask important questions and to demand answers so that we can respond and ensure that we have an equitable system of policing in L.A. County.’’

Sheriff Jim McDonnell, who served on the Citizens’ Commission on Jail Violence before being elected to his post, opposes subpoena power for the watchdog group. McDonnell is up for re-election next month.