With union leaders hesitant to endorse Democratic frontrunner Hillary Clinton, a coalition of their members are preparing a national Labor Day rally in support of her rival, Bernie Sanders.

Throughout the Democratic primary, Hillary has been able to hold her lead. According to polls, though, Sanders has been quickly narrowing the gap. At the same time, Hillary has struggled to gain traction with the labor movement. She is also losing support among local unions and members.

Labor for Bernie argues the Vermont socialist is the best candidate for working Americans. At Labor Day gatherings across the country, supporters of the group plan to pass out flyers, hold signs and march in parades. It also claims to have over 7,000 union members sign its letter of support.

Former Communications Workers of America President Larry Cohen is helping to lead the group. Cohen, a long time and influential union leader, announced his support for Sanders in July. Though many local unions were already backing him, Cohen became the first major national leader to do so.

“By showing the depth of grassroots support for Bernie, more local and national labor leaders will seriously consider his candidacy,” Cohen said in a statement provided to The Daily Caller News Foundation.

Cohen plans to participate in a rally in eastern Iowa on Sunday and Monday. Labor for Bernie also plans to hold events in Los Angeles, Atlanta, Boston, Cape Cod and Detroit, along with New York state, Texas, Vermont and Wisconsin.

Unions are a powerful influence in politics. They aren’t just major financial contributors, they can also rally supporters. Even with the support among many within the labor movement, union leaders fear he is not electable. Labor Day is an important holiday for unions. It isn’t just a celebration of American workers, it has been used to celebrate the labor movement and the history of unions.

Labor for Bernie has listed unions that have already endorsed or recommended Sanders to their members. This includes local chapters of the American Federation of Teachers, American Postal Workers Union, Communications Workers of America, the International Brotherhood of Electrical Workers, United Brotherhood Of Carpenters, AFL-CIO, Vermont State Labor Council. The national United Electrical Workers and the National Nurses United have also decided to back them.

It was the recent trade debate that first helped Sanders begin to edge Hillary out for union support. The Trans-Pacific Partnership (TPP) is currently being negotiated between President Barack Obama and the leaders of 11 other Pacific nations.

Though the labor movement adamantly opposed the deal , Hillary was reluctant to take a firm stand against it. Critics have claimed the deal will benefit corporations and special interests at the expense of working Americans and the environment.

In contrast, Sanders was much more adamantly opposed to the deal. He has also made issues important to the labor movement as central part of his campaign. In July, Sanders introduced a bill that would raise the federal minimum wage to $15 an hour.

The AFL-CIO hosted a meeting in July as part of an ongoing effort to determine which candidate to endorse. The union instead decided to delay endorsing anyone. Other labor groups are holding meetings or other initiatives to figure out who to back.

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