Manning, who filed official paperwork on Thursday, is one of four Democratic candidates who have filed for the Senate seat. She is running as a Democrat.
She is the fourth candidate - and third Democrat - to file as a challenger to Cardin, the Democratic senior senator from Maryland, who is expected to run for a third term this year.
Manning was arrested in Iraq in 2010 for sharing more than 700,000 confidential documents, videos and diplomatic cables with anti-secrecy website Wikileaks.More news: Wintry mix expected through Friday morning
More news: Trump Says He'd Beat Oprah If She Runs For President
More news: Facebook modifies news feed to bring family, friends closer
Manning refused to make any comment regarding her filing but said she would release a statement in the coming days. "On Tuesday, which was National Law Enforcement Appreciation Day, she stirred controversy when she tweeted ". the police", and then, "police kill hundreds of people every year with absolute impunity". While Manning is originally from Oklahoma, Cardin has deep ties in Maryland, where he served in the Maryland House of Delegates for 20 years before taking the seat of retired Sen.
Manning's high-profile leak drew media coverage around the world.
She was placed in solitary confinement after attempting suicide twice. She has been lauded as a hero by some on the left but also decried by many, including Trump, who called her an "ungrateful traitor" on Twitter.
During her incarceration Manning battled for, and won, the right to start hormone treatment. Ben Cardin, D-Md., speaks during a Senate Foreign Relations Committee hearing on.