As Democratic voters head to primaries in five delegate-rich states on Tuesday, Bernie Sanders is riding a wave of momentum based on his call for a political revolution and an economic policy platform he says puts working people and the middle class above the interests of multinational corporations and the very wealthy. On that score, his campaign received a major boost on Monday when the nation's largest union of transportation workers, the Amalgamated Transit Union, officially endorsed Sanders for president.

"His unabashed support of civil rights, public services, free tuition at public colleges, increases in Social Security and the minimum wage, make him an ideal candidate." —Lawrence Hanley, Amalgamated Transit Union

"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," said ATU International President Larry Hanley in making the announcement.

With primaries in Ohio, Missouri, Florida, Illinois, and North Carolina Tuesday, the ATU—which is now the fourth national union to endorse Sanders—said it will be mobilizing across the country as it urges members to put their support behind the Sanders campaign in all the states which have yet to vote. With more than 192,000 members in 253 local unions spread across 47 states and nine Canadian provinces, the ATU represents bus drivers, light rail operators, maintenance and clerical personnel and other transit and municipal employees.

In addition to his historic support for workers and organized labor, Hanley listed a range of issues which fueled the ATU's endorsement of Sanders.

"His unabashed support of civil rights, public services, free tuition at public colleges, increases in Social Security and the minimum wage, make him an ideal candidate. But the Labor Movement owes Senator Sanders so much for his consistent opposition to right wing programs and his championing of first-rate healthcare for all – which is already the global standard," Hanley said.

"Our political revolution can only happen if thousands of working people stand together and tell the millionaires and billionaires they can’t have it all." —Bernie SandersSanders welcomed the news.

"I’m proud to have the support of the Amalgamated Transit Union," Sanders said in a statement on Monday. "Our political revolution can only happen if thousands of working people stand together and tell the millionaires and billionaires they can’t have it all."

Though Hillary Clinton has so far won a larger number of national labor endorsements, Sanders has made steady gains by winning the broad support of rank-and-file union members, smaller locals, and unaffiliated workers. Last week in California, four separate UNITE HERE locals—representing nearly 20,000 hotel, casino and food service workers in the northern part of the state—endorsed Sanders. Workers in those local chapters explained why their votes were going to Sanders when their state—the most populous in the nation—holds its primary on June 7.

"Bernie Sanders is the best candidate for working people and the middle class," said Robert Flatt, a cook at a casino Lincoln, California and member of UNITE HERE Local 49. "He believes in equal pay for women, expanding Social Security and raising the minimum wage. He supports bills that make it easier for workers to form unions and have a voice in their workplace."

And Johnny Stake, a stand worker at the Oakland Coliseum and member of UNITE HERE Local 2850, added, "As a working-class African-American man, I’m supporting Bernie Sanders because he’s the only one who is talking about fundamental change in our economic system. We cannot continue to accept incremental increases in our income. We need a qualitative leap – or else we’ll be doomed to continue living paycheck to paycheck. Plus, I knew Bernie was cool when I saw a picture of him when he was younger, protesting in the streets for civil rights for black people."

As the ATU's Hanley said, putting this year's presidential election in historic context, "This is no ordinary time in U.S. history and our nation is crying out for a leader who owes nothing to the corporate interests responsible for undermining the American middle class. Our executive board recognizes what’s at stake in this election and have made the bold decision of endorsing Bernie Sanders for President. Bernie is right for working people and right for America."