Cardin, the incumbent senator Manning is challenging, has been a USA senator since 2006.
Chelsea Manning, the transgender soldier who was convicted of giving classified information to WikiLeaks, has filed paperwork with the Federal Elections Commission for a U.S. Senate bid in Maryland. Maryland is considered a safely Democratic state for this year's Senate race.More news: Liverpool boss Jurgen Klopp reveals why he snubbed Man Utd offer
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Manning was convicted in 2013 of stealing 750,000 pages of sensitive government documents and videos and leaking them to WikiLeaks and received a sentence of 35 years in prison on 20 counts, including violating the Espionage Act. Manning pleaded guilty to some charges but was acquitted of the most serious charge brought against her: aiding the enemy. In an exclusive 2017 interview with ABC's Juju Chang for a special edition of Nightline, Manning said, "My intention was to draw attention to this ... and do the right thing".
A representative for Manning did not immediately respond to a request for comment.
Known as Bradley Manning at the time of her 2010 arrest, Manning came out as transgender after being sentenced.