Manning drew national attention when she filed in January to run against the 74-year-old Cardin.
The 30-year-old government whistleblower and former intelligence analyst began her race with some momentum, but her campaign stalled in recent weeks as appearances and fundraising declined, The Washington Post reported.
Manning conceded with an online statement Tuesday addressing her pullback in campaigning efforts to focus on her physical and mental health in recent months. She thanked her staff and volunteers for their work while telling her supporters to realize the “power we have as individuals.”
“We started this campaign knowing it was a long-shot,” Manning wrote in her concession. “But, after spending hours and hours knocking on doors and making phone calls, I’m convinced that the change people truly need goes beyond what our corrupt two-party system is willing to offer.”
Last month, Manning posted a thread on Twitter claiming that Maryland residents were uninterested in voting and that “primaries are rigged.”
In one of her tweets, Manning said that “we need something radically different and we cant just ask for it or expect it to happen somehow.”
Cardin, long a fixture on the Maryland political scene, is a heavy favorite to win a third Senate term in November’s general election in the strongly Democratic state.
Manning, in her campaign, called for abolishing the U.S. Immigration and Customs Enforcement agency months before it was put in the spotlight by President Donald Trump’s since-rescinded policy of cracking down on undocumented immigrants by separating parents from their children at the U.S.-Mexico border.
“We don’t need ICE,” she said in an interview with Democracy Now! in March. “We don’t need a lot of these gigantic... police agencies that are singularly focused on deporting people.”
Manning, a former Army private, pleaded guilty in 2013 to leaking more than 700,000 government files to WikiLeaks. Manning was sentenced to 35 years in prison, but was released in May 2017 after former President Barack Obama commuted her sentence in January of that year, just as he was leaving office.
She became a symbol of transgender rights while in jail when she went on a hunger strike to protest officials who denied her request for gender confirmation treatment. Manning, who was assigned male at birth and transitioned during her imprisonment, ended her strike after the Army complied with her request.
This story has been updated with a concession statement from Manning.