Fayard announced her backing of Campbell on Tuesday in a statement, saying she supports the state utility regulator in the Dec. 10 runoff against Republican state Treasurer John Kennedy.
Fayard described herself as "happy to stand with Democrats across our state and country in support" of Campbell.
The competition between Fayard and Campbell had been particularly bitter ahead of the Senate primary election earlier this month.
Campbell criticized Fayard's family, her experience and her wealth. Fayard in return sought to tie Campbell to Duke, the former Ku Klux Klan leader who also ran unsuccessfully for the Senate seat.