Democratic U.S. presidential candidate Hillary Clinton (L) listens to Senator Bernie Sanders speak during a Democratic debate hosted by CNN and New York One at the Brooklyn Navy Yard in New York April 14, 2016. REUTERS/Lucas Jackson

One of the major labor unions to endorse Sen. Bernie Sanders (I-Vt.) during the Democratic primary announced Monday that it is endorsing former Secretary of State Hillary Clinton for president, now that she has effectively locked up the nomination.

“We know that elections are about choices,” the Communications Workers of America, which represents 700,000 workers, said in a statement. “The contrast between Hillary Clinton and Donald Trump, the presumptive Republican nominee, couldn’t be greater … CWA will do everything we can to mobilize our members and activists to elect Hillary Clinton President of the United States.”

In December, bucking the pro-Clinton trend among unions, CWA chose to throw its weight behind Sanders, citing his early opposition to the Trans-Pacific Partnership and his support for a $15 federal minimum wage. But like the rest of organized labor, the union is now primarily concerned with keeping Donald Trump out of the White House at all costs.

Accordingly, CWA has gotten in line with Clinton and will put its resources behind the presumptive nominee. The union said it will staff phone banks and do canvassing for Clinton, and said that it expects to maintain a heavy presence in the swing state of Ohio.

The announcement raises the question of whether other pro-Sanders unions will declare their support for Clinton ― particularly if Sanders endorses his opponent, perhaps imminently, or when Clinton’s nominee status becomes official later this month at the party’s convention in Philadelphia.

A handful of other national unions have endorsed Sanders over Clinton, including National Nurses United, the American Postal Workers Union and the Amalgamated Transit Union, along with a number of smaller union locals. Most major unions, however, have said they will be supporting Clinton, long considered the more likely nominee.

NNU has been perhaps the most vociferous backer of Sanders, with the union’s nurses a constant presence at the self-described democratic socialist’s rallies. When asked whether NNU plans to endorse Clinton now that she has effectively secured the nomination, union spokesman Chuck Idelson simply said that “there is no change in our position.”

APWU and ATU could not immediately be reached for comment on Monday.

Just because their candidate is unlikely to win the nomination doesn’t mean the unions’ endorsement of Sanders would be a lost cause. After all, they chose to back Sanders because they preferred his policy positions to Clinton’s. Their public support helped make his presidential run more viable, thereby altering the course of the primary and Clinton’s candidacy.

It’s clear now that Sanders’ policy platform has influenced the Democratic Party’s official agenda in some ways that are dear to unions. The party is expected to support a $15 national minimum wage proposal, which Clinton herself has been unwilling to do. The $15 wage floor has been one of the pillars of the Sanders campaign.

The Clinton campaign said Monday that it was happy to have CWA’s endorsement. In a statement, Clinton said she was “honored” to have the union’s backing, and she pledged to “defend American jobs and American workers.” She reiterated her opposition to the TPP “before and after the election.”