Photo

The Service Employees International Union, one of the largest and most politically influential unions, threw its support behind Hillary Rodham Clinton on Tuesday, the latest indication that she is consolidating support among the institutional pillars of the Democratic Party.

The national board of the S.E.I.U. voted overwhelmingly to back Mrs. Clinton, offering her the support of a union that has been at the forefront of the push for higher wages and also taken a leadership role in the broader social justice movements animating the left. The endorsement offers Mrs. Clinton additional grass-roots support and also a symbolic statement as she fends off a populist threat from Senator Bernie Sanders of Vermont.

Mary Kay Henry, S.E.I.U’s president, said the group’s members “believe she is going to fight like hell for our agenda.”

Ms. Henry, noting that Mrs. Clinton had met twice with S.E.I.U. workers this year, cited the former secretary of state’s support for higher wages, an immigration overhaul, strengthening voting rights and addressing mass incarceration. Mrs. Clinton also assured the group that, if elected, she would use her executive authority to bolster union protections, Ms. Henry said.

“She knows that workers being able to join together and collectively bargain is essential to building an economy that works for everybody,” the service workers president said.

Mrs. Clinton has now won the support of some of the largest unions in the country, having previously been backed by the two groups representing teachers, the National Education Association and the American Federation of Teachers, as well as the American Federation of State, County and Municipal Employees, or Afscme.

The service employees union, which has about two million members, offers Mrs. Clinton support from a group that is about half female and includes many minorities, demographic groups she is counting on to capture the Democratic nomination. With a growing number of Hispanics joining its ranks, the group has increased its activism in recent years around immigration issues. It has also gotten involved in racial justice issues, lending its support to the Black Lives Matter movement.

Notably, the service employees have been leading advocates in the Fight for $15 push to raise the minimum wage to $15 an hour for fast food employees. Recently, though, workers from a broader array of industries have joined the effort to the lift the wage. In Saturday’s debate, Mrs. Clinton came under attack from former Gov. Martin O’Malley of Maryland for only supporting an increase of the overall minimum wage to $12 per hour.

“We don’t see it as a contradiction,” Ms. Henry said about the union’s support for a candidate who is not supportive of raising the overall wage to $15, adding that Mrs. Clinton had encouraged her to keep up the pressure to push the wage to that level. “She said to me, ‘Listen, S.E.I.U. and Fight for $15 should continue to push the whole nation, we all need to get to $15.’”

The union’s support is a turnabout from the 2008 presidential race. The group delivered an important endorsement to Senator Barack Obama in February of that year, in the middle of his battle versus Mrs. Clinton, which bolstered his support at a moment when the outcome of the race was still in doubt.