Manning, who was granted clemency by former US President Barack Obama, was released in May from a US military prison in Kansas where she had been serving time for passing secrets to the WikiLeaks website in the biggest breach of classified data in the history of the United States.
Democratic Senator Ben Cardin was elected in 2006 to that seat and is expected to run for re-election this year. He is the senior Democrat on the Senate Foreign Relations Committee.
Cardin was easily re-elected in 2012, beating his Republican challenger by 30 points in the heavily-Democratic state.
She came out as transgendered shortly after her sentencing, but the military denied her request for hormone therapy treatment while behind bars. She was placed in solitary confinement after attempting suicide twice.