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Bernie Sanders just gave Hillary Clinton something to worry about.

Scott Elsen,Getty Images

The American Workers Postal Union on Thursday backed Sanders, citing his defense of the U.S. Postal Service, along with his commitment to a higher minimum wage and expanded Social Security, as its reasons for the endorsement. It brings his total number of union endorsements to two—AWPU and National Nurses United. 

Clinton and Sanders have been in a not-so-subtle battle for union support, with Sanders banking on his long record of defending labor in the Senate. and Clinton notably coming out against Obama’s Trans-Pacific Partnership (TPP), which unions despise.

So far this cycle, Clinton has been rewarded with the support of some of the biggest unions, including the National Education Association (NEA) and the American Federation of State, County, and Municipal Employees (AFSCME). With Clinton still maintaining a sizable lead over Sanders as they head into the second presidential debate on Saturday, AWPU could help give the latter candidate a boost.