 -- Bernie Sanders has landed his biggest labor endorsement to date: the American Postal Workers Union, or APWU.

The group represents 200,000 U.S. Postal Service employees and retirees from across all 50 states. The APWU executive board announced its choice early this morning.

The commitment comes at a critical moment for the Sanders campaign, which watched Hillary Clinton snag endorsements in October from the first- and third-largest unions in the nation: the National Education Association (NEA) and the American Federation of State, County and Municipal Employees (AFSCME).

Their combined membership totals more than 4 million, far outweighing the 190,000 members of National Nurses United, Sanders’s only major union endorsement before today.

“Politics as usual has not worked. It’s time for a political revolution,” Postal Workers President Mark Dimondstein said in a statement, invoking the battle cry of the Vermont senator. “Bernie Sanders stands above all others as a true champion of postal workers and other workers throughout the country. He doesn’t just talk the talk. He walks the walk.”

Much of the talking revolves around post office closures across the nation, particularly in rural areas, and new ways to raise postal revenue. Dimondstein added that the group had been impressed when Sanders spoke to postal workers in Las Vegas, which was his first stop after October’s debate.

While Sanders has only received endorsements from two sitting members of the U.S. Congress, labor was supposed to be his sweet spot. Progressives have cheered his appearances at picket lines, his calls for a $15 minimum wage and his early opposition to the Trans-Pacific Partnership. But so far, unions have seemed skittish about turning their backs on a dominant front-runner like Clinton.

This hasn’t stopped some progressive rank-and-file members from breaking for Sanders already. Before the APWU had made its decision official, the Sanders camp rolled out its New Hampshire steering committee, featuring several postal workers and mail carriers.

“Hillary Clinton, I’ve met her,” said John Joyal, a training instructor at the Portsmouth Naval Shipyard in Maine and one of the steering committee’s 53 members. “I’ve taken pictures with her. But she’s too wishy-washy. Bernie’s got my support, no two ways about it.”

Joyal added that union brass might be tempted to swing their votes toward Clinton, but that dues-paying members in the “Live Free or Die” state of New Hampshire will not necessarily follow. “A lot of these members won’t be told who to vote for,” he told ABC News.

Sanders hopes that other unions will follow but, in the meantime, the scope of this new endorsement is a statement of its own.

"Postal workers are in every state, every territory, and the District of Columbia," APWU spokesman Jamie Horowitz told ABC News. "They're in almost every ZIP code. So it’s truly a national endorsement."