US Democratic presidential front-runner Joe Biden has won the backing of the National Education Association, the largest US teachers union with 3 million members.

"Joe is the tireless advocate for public education and is the partner that students and educators need in the White House," NEA President Lily Eskelsen García said in a statement on Saturday. "He understands that we have a moral responsibility to provide a great neighborhood public school for every student in every ZIP code." The endorsement comes days before Biden competes against US Senator Bernie Sanders in Democratic presidential primary contests on Tuesday in Florida, Illinois, Ohio and Arizona.