The conservative media organisation Red Maryland first reported Manning's intention to run.
Manning told The Washington Post at the weekend that she might release more information in the coming days.
Cardin spokeswoman Sue Walitsky did not directly address Manning's candidacy or anyone else's. But she said that "Cardin is looking forward to a vigorous debate of the issues and a robust conversation with Maryland voters."
Manning would not be the first transgender candidate to challenge a sitting member of Congress.
Kristin Beck, a retired US Navy Seal who is transgender, failed to unseat US Representative Steny Hoyer in Maryland's Democratic Primary in 2016. Beck got 12 per cent of the vote.
"It was too much for me to run for Congress," Beck said in a phone interview. "I should have run for something lower (state or local office). She might as well be running for president."
Beck said Manning is pulling a "publicity stunt."
"I totally, 100 per cent disagree with everything she did," Beck said.
"She's just grabbing headlines. I know what that feels like. I've been in the headlines. You get a sense that you're worthy and doing something that counts. And when you lose that, you try to do something to grab a headline."
Manning had said she wanted to expose what she considered to be the US military's disregard of the effects of war on civilians and she released information that she didn't believe would harm the US.
Obama's decision to commute Manning's sentence to about seven years drew strong criticism from members of Congress and others, with Republican House Speaker Paul Ryan calling the move "just outrageous".
When Manning was released from Fort Leavenworth, Kansas, in May, she told AP in an email that she was eager to define her future — but made no mention of politics.
"I'm figuring things out right now — which is exciting, awkward, fun, and all new for me," Manning wrote.
The Oklahoma native had planned to move to Maryland, where she has an aunt. She has been registered to vote at the apartment in North Bethesda since mid-August, according to the Maryland State Board of Elections.
Her recent move would not affect a Senate bid. To run for that office, a person must be at least 30 years old, a US citizen for nine years and an inhabitant of the state at election time, according to the Maryland State Board of Elections. A felony conviction does not appear to preclude a run.
Manning is yet to file for the primary with the state elections board, which she must do in person by February 27, according to the board's website.
Cardin is also yet to file. But campaign finance reports show that his organisation had nearly $2 million cash on hand in late September.
- AP
