Still, the service employees and teachers unions could provide valuable aid in organizing get-out-the-vote efforts on the Warren's behalf. With memberships each in the tens of thousands, both could prove pivotal to bulking up the Senate hopeful's field operation.

Warren, who served as an adviser to President Obama and was instrumental in the creation of the president's new consumer financial protection agency, warmly welcomed the endorsements.

"I am delighted to receive the endorsement of SEIU's 75,000 members," Warren said in a statement. "I've worked closely with SEIU for years now — advocating for working families and fighting to get a Consumer Financial Protection Bureau created. I look forward to continuing to work with them to improve the lives of working men and women here in the Commonwealth and across the country."

Warren attended a candidate forum on Saturday with the service workers that Brown elected to skip.

Massachusetts Teacher Association President Paul Toner credited Warren's history as an educator in his endorsement.

Elizabeth WarrenElizabeth Ann WarrenTrump's SEC may negate investors' ability to fight securities fraud Schatz's ignorance of our Anglo-American legal heritage illustrates problem with government Dems ponder gender politics of 2020 nominee MORE has the right values and vision to stand up for educators and students in Congress,” Toner said in a statement. “She understands the issues facing teachers because she was one. Elizabeth Warren recognizes just how important good teachers are to the future of our families, communities, and our country and she will fight to create new opportunities for all of our students from preschool to graduate school.”

Massachusetts Republicans, meanwhile, were highlighting criticism leveled at Warren by a state-level Democratic official upset that she had skipped town for candidate forums.

"She’s too good for everybody. This is the feeling that I get. This is very elitist to me to when I hear that she’s too busy,” said Maura Flynn, Maynard, Mass.'s town committee chairman.

Republicans seized on the opportunity to paint Warren as "elitist," a theme they have cultivated throughout the race, often referring to Warren's ties to Harvard and the Obama administration.

“Don’t take our word for it that Professor Warren is an elitist — even her fellow Democrats know she thinks she is smarter and better than everyone else," said Alleigh Marré, a spokeswoman for the Massachusetts Republican Party.