Democratic leaders mustered labor union support on Monday as hundreds of supporters cheered on Democratic Senate candidate Elizabeth Warren at a Dorchester rally, where Sen. John Kerry, Warren and union members criticized Republican assaults on workers.

“I’m proud to be campaigning with you to build power for working people and to make Elizabeth Warren the next senator for this state,” said Richard Trumka, president of American Federation of Labor and Congress of Industrial Organizations, to the audience.

The rally was held at the Local International Brotherhood of Electrical Workers 103.

Various labor unions and organizations, such as Teamsters Local 25 and Painters and Allied District Council 35, attended with banners announcing support for Warren.

“I am not in this race to go down to Washington so I can figure out how to help more millionaires, more billionaires and big oil,” Warren said. “That’s not why I’m in this race. I am in this race to help level the playing field. I am in this race to fight for working people.”

Rep. John Tierney and Fourth District congressional candidate Joe Kennedy III, both Democrats, also spoke at the rally.

Democratic senator and 2004 presidential candidate Kerry attended the rally to support the Democratic candidates.

“Vote after vote, [Republicans] engaged in [the] greatest assault on organized labor I’ve ever seen,” Kerry said. “[This race] will decide the agenda for the country.”

Warren said she plans to focus on rebuilding the middle class.

“I am here today because unions helped build America’s middle class,” she said. “We will rebuild America’s middle class because that stuff we know is the future of America.”

Trumka said Warren is the best choice for laborers and labor unions.

“We all get one vote on election day, just like the billionaires, and we’re going to cast votes together for Elizabeth Warren,” he said. “Warren is the champion of working people, and we have the power to send her to Washington.”

Speakers criticized Republican U.S. Sen. Scott Brown for frequently voting with the Republican party.

“He’s had a few good votes in there, but he’s had a lot of bad votes in there,” Warren said. “The president proposed a jobs bill that would have supported 22,000 jobs here in Massachusetts. Scott Brown joined with every other Republican to vote ‘no.’”

But Brown was one of the three Republican senators who actually voted for the Dodd-Frank Wall Street Reform and Consumer Protection Act, according to FactCheck.org.

Brown has also veered from Republicans in some of his other views, including his support of abortion rights, and recently opposed Romney’s “47 percent” remark.

But Trumka said Brown advertises himself as a moderate Republican and a simple man, but has changed his colors to blend in with the right wing.

“The new Scott Brown votes every time with the 1 percent and Tea Party,” he said.

Warren said she supports unemployment benefits for struggling Americans.

“Scott Brown has voted against the extension of unemployment benefits 16 times,” Warren said. “I will never turn my back on our brothers and sisters, on our neighbors and friends, when they hurt.”

Boston Mayor Thomas Menino attended the rally after publicly endorsing Warren on Friday.

Some people who attended the rally said they supported Warren as the best choice for labor unions.

“She’s fantastic,” said Michael McGrath, a Teamsters Local 25 member from Woburn. “I’m a union man. I don’t care how good they look — you’ve got to go Democratic. The Republicans will sell you down the road any day of the week.”

Kathy Collins, an IBEW member from Springfield, said reelecting Brown would be damaging to the working class.

“He made promises to be an independent voice in Washington, and he has not,” she said.

Doug Henry, a Teamsters Local 25 member from Hull, said Brown was frequently unsupportive of the working class.

“[Warren] stands for all the right things,” he said. “Scott Brown has voted against the working class virtually every chance he’s had. He’s made a few good votes, but I’m sure it was only when he had permission from [Senate Minority Leader] Mitch McConnell.”

Despite these sentiments, a Sept. 17 University of Massachusetts and Boston Herald poll found Brown leading 49 percent to 45 percent.

Fifty percent of viewers said Brown won the first debate last Thursday, while only 40 percent thought Warren won, according to a poll by Kimball Political Consulting, a right-leaning firm.