(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."
"Bernie's issues align with nurses from top to bottom," DeMoro said, noting that Sanders earned the groups support because of his positions on trade, minimum wage and expanding Social Security and Medicare."
Earlier this year, the group conducted an internal poll to decide who to endorse. Charles Idelson, the group's spokesman, said the Vermont senator won "overwhelming support," but no percentage was given.
Another factor in the group's decision: Sanders' speech to the AFL-CIO Executive Council last month, according to Idelson.
Sanders, along with former Secretary of State Hillary Clinton and former Maryland Gov. Martin O'Malley, spoke before the AFL-CIO executive council last month. The speeches were closed, but provided the candidates with an opportunity to pitch themselves to the large union organization.
"He made it clear that you will never have to wonder which side he is on," said DeMoro, a member of the AFL-CIO executive council. "As he told the union leaders, 'I see myself as part of you. This is not a conventional moment, we are fighting for the future of this country.'"
Most members of the AFL-CIO have held off endorsing in the Democratic primary.
Clinton scored the first major union endorsement in July when the American Federation of Teachers endorsed the Democratic frontrunner.
Randi Weingarten, the AFT president, is a Clinton supporter and serves on the board of the main pro-Clinton super PAC.