Sanders, an independent who describes himself as a "democratic socialist," will follow a statement with a major campaign kickoff in his home state in several weeks. Two people familiar with his announcement spoke to The Associated Press under condition of anonymity to describe internal planning.

Sanders will become the second major Democrat in the race, joining Clinton. He has urged the former secretary of state to speak out strongly about issues related to income inequality and climate change.

The former first lady and New York senator is viewed as a heavy favorite in the primary and entered the race earlier this month.