SAN FRANCISCO — New Jersey Sen. Cory Booker stood out from the large field of 2020 Democratic presidential contenders Saturday by being the only candidate at the California Democratic Convention to tackle head-on the mass shooting in Virginia Beach, Virginia.

Twelve city government employees were killed by a co-worker Friday, a horrific event mentioned only in passing at the convention, even as polls show gun safety is a key voting issue for many Democrats.

As the final speaker on a daylong program of 11 presidential contenders, Booker hit the issue hard.

"We are seeing the normalization of mass murder in our country," Booker said, his voice rising to a near shout. "It is time for us as a nation not to normalize the violence and the culture of gun violence.

An attendee wears a U.S. flag tie with pins during the California Democratic Convention in San Francisco, California on Saturday. STEPHEN LAM

"It is time that we come together and stand together and take the fight to the NRA and the corporate gun lobby like we have never seen before. We can lead that fight and we can win."

With his passionate speech, Booker managed to capture the attention of a crowd that had often been restless and noisy during other candidates' turn at the podium. When he concluded, invoking the murder of Martin Luther King Jr. as "a challenge to our generation," people in the audience jumped to their feet in a standing ovation — the first true breakout moment of the day.

It was a longtime coming for Booker, who has languished in the low single digits in most polls as many Democrats desperate to defeat President Donald Trump next year have been slow to embrace Booker's optimistic message of unity.

"Booker really needed that speech," said Lara Blakely, a convention delegate and the former mayor of Monrovia, California. "He needed to do something to get people to give him another look."

Booker, had prepared a different speech for the convention but discarded it amid news of the Virginia Beach massacre, aides said. On Saturday morning, he began writing a new one. Much of it was delivered extemporaneously, they said.

Other contenders at the convention faced a mixed reception from the assembled delegates.

Supporters of Democratic presidential candidate and Mayor of South Bend, Indiana, Pete Buttigieg chant during the California Democratic Convention in San Francisco on Saturday. STEPHEN LAM

California Sen. Kamala Harris, under pressure to deliver to her home-state crowd, drew applause in an otherwise low-key speech in calling for Trump to be impeached.

Massachusetts Sen. Elizabeth Warren made the case for herself as a proven winner, as did Minnesota Sen. Amy Klobuchar.

Former Texas Rep. Beto O'Rourke and South Bend, Indiana, Mayor Pete Buttigieg mainly stuck to their message of generational transformation. Buttigieg, who is gay, also spoke proudly of his marriage to his husband, Chasten.

"This morning I woke up next to my husband by the grace of a single vote in the Supreme Court," he said.

Colorado Gov. John Hickenlooper, meanwhile, staked out a centrist position in front of the largely progressive crowd. "Socialism is not the answer," he declared, drawing boos.