WASHINGTON—Congress struck a tentative bipartisan agreement that would authorize 12 weeks of paid parental leave for all federal workers, in a potentially historic deal negotiated with the White House.

Draft language for a must-pass annual defense policy bill includes a provision that would allow 2.1 million civilians who work for the U.S. government across the country to take paid leave to care for a new baby after birth, adoption or the initiation of foster care, according to multiple people familiar with the agreement.

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