(CNN) National Nurses United -- a 185,000-person union and the largest group of nurses in the country -- endorsed Bernie Sanders at their national conference Monday.

The endorsement is significant because it is Sanders' first sizable union endorsement in his quest for the Democratic nomination and comes after Sanders and other Democratic candidates pitched themselves to the AFL-CIO, a group that includes the nurses union, last month.

"Bernie Sanders has a proven track record of uncompromised activism and advocacy for working people, and a message that resonates with nurses, and, as we have all seen, tens of thousands of people across the country," NNU Executive Director RoseAnn DeMoro said in a press release announcing the endorsement. "He can talk about our issues as well as we can talk about our issues. We are proud to stand with him in his candidacy for President today."

The nurses organization officially announced their support at an event with Sanders in Oakland, California. The group, which is 90% women, called the meeting a "Brunch with Bernie."

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