A national transit workers union threw its weight behind presidential hopeful Sen. Bernie Sanders on Monday, endorsing the Vermont independent over Democratic front-runner Hillary Clinton.

The Amalgamated Transit Union waited longer than most major unions before endorsing a candidate in the race for the White House. Union leadership said it had carried out a careful and deliberative process with union members before deciding to side with Sanders.

“The sincerity of Bernie Sanders and his long standing fidelity to the issues that are so important to working people are what convinced us that standing with Bernie is standing with the 99% of America that has been left out of the mainstream public debate, cheated out of our jobs and denied the true meaning of the American dream,” Larry Hanley, the union's president, said in a statement.

Since announcing their candidacies, Clinton and Sanders have had a long and protracted battle for the backing of organized labor. By any measure, Clinton is way out in front of the self-described democratic socialist. In October, she locked up endorsements from unions representing the majority of unionized workers in the U.S., including the largest public sector labor groups in the country.

But a handful of progressive unions have broken with the pack and declared their support for Sanders. That includes the Communications Workers of America, National Nurses United, the American Postal Workers Union, and now the ATU.

Most important for Sanders, those unions' backing has helped stave off a potential endorsement of Clinton by the AFL-CIO, the national labor federation they are all members of.

Many unions came out in favor of Clinton long before Sanders became a viable threat for the Democratic nomination. Before ATU made its endorsement, Hanley told The Huffington Post that union leadership had made a conscious decision to wait before backing any one of the candidates.

"We think campaigns matter," Hanley said. "We think that until people campaign, you don't get a good sense of how they'll govern. In fairness to all the candidates and our members, we thought it was important that we wait and watch and hear what the candidates have to say and how they come out on positions."

Though union membership has dwindled significantly, labor unions remain a pillar of the Democratic base. Democratic candidates perennially rely on unions to make phone calls, knock on doors and generally steer financial support to their campaigns. The Citizens United ruling that allows corporate America to spend unlimited amounts on political campaigns also applies to unions, though organized labor's coffers aren't nearly as deep and rely primarily on member dues.

ATU represents 190,000 members working mostly in bus and rail jobs. As of December, the union's political action committee had raised $1 million for this election cycle, according to data from the Center for Responsive Politics. The union's contributions have gone overwhelmingly toward Democrats.