Her candidacy brings drama to an otherwise staid primary and gives Republicans an opportunity to stoke divisions among Democrats. But analysts say Manning's maverick approach may not play well in Maryland, a progressive but establishment state that is home to the National Security Agency and went for Hillary Clinton over Bernie Sanders by a large margin in the 2016 Democratic presidential primary.
"Challenges like this rarely succeed around the country, much less in Maryland," said John T. Willis, a political-science professor at the University of Baltimore. "Maryland is a fairly strong party state, and the political culture is one that rewards experience and familiarity in the political world."
Manning's unlikely entry into the race could draw a different kind of national attention to Maryland in a year when Democrats are trying to unseat Gov. Larry Hogan (R), who has distanced himself from Trump, the GOP's biggest electoral liability in a blue state.
For the far-left wing of the party, Manning's felony conviction and sentence, which was commuted by President Obama last Jan. 17, do not disqualify her. In fact, the opposite may be true.
"Folks that are applying litmus tests for candidates based on criminal record and experience with police — that's one of many reasons why we don't have the diversity of representation that we need in the U.S. Senate," said Neil Sroka, spokesman for the national progressive group, Democracy for America.
"We were glad to see she's running and we'll be watching that race," Sroka said. "When an incumbent is challenged by a strong challenger that inevitably makes that incumbent more accessible to the Democratic base."
But Democracy for America will wait to see all the Senate candidates in action before making any endorsements, Sroka said.
The organization is backing former NAACP president Ben Jealous for Maryland governor and former Maryland congresswoman Donna F. Edwards for Prince George's County executive.
State Democratic officials declined to comment about Manning's candidacy.
Groups that have worked to elect LGBTQ candidates elsewhere — such as the Victory Fund, which backed Danica Roem in her successful quest to win a seat in the Virginia legislature — also passed on a request to wade into the supercharged politics around Manning.
"She has not applied for endorsement and we would have to review the viability of her campaign, positions on LGBTQ equality and reproductive rights, etc.," Victory Fund spokesman Elliot Imse said in an email. "We are not yet ready to comment on her candidacy."
The conservative blog Red Maryland broke the news this weekend that Manning filed the federal documents required for her to begin raising money for the race.
She must still file in person with the Maryland Board of Elections to have her name placed on the primary ballot. The deadline is 9 p.m. Feb. 27.
Three other, largely unknown Democratic hopefuls have filed with the state: Jerome Segal of Montgomery County, Richard Vaughn of Baltimore County and Debbie Wilson of Charles County.
No Republicans have filed yet. But that didn't stop Maryland Republican Party Chairman Dirk Haire from challenging Kathleen Matthews, the state Democratic Party chair, to call on Manning to withdraw.
"I don't think someone who is a convicted traitor possesses the judgment to be an effective senator," Haire said.
Matthews did not respond to messages.
Sen. Cardin, whose last campaign motto was "My friend Ben," is expected to run on a mix of local and national issues including the restoration of Chesapeake Bay cleanup funds and calls for investigation of Russian meddling in U.S. elections.
Manning is far better known than past Cardin challengers, such as state Sen. C. Anthony Muse (D-Prince George's) in 2013. And cable news coverage of a Manning run could make it difficult for Cardin to brush off the competition.
But Willis noted that notoriety, celebrity and even a strong grass roots following is not necessarily enough to compete against veteran political leaders.
For example, he said, the late entrance of Black Lives Matter activist DeRay Mckesson into the 2016 Democratic primary for Baltimore mayor caused a stir, but Mckesson finished sixth in the race.
Todd Eberly, a political-science professor at St. Mary's College, noted that Cardin is well respected among the many federal government and defense workers who live in Maryland, making it difficult for Manning, "whose claim to fame is espionage," to gain traction.
"If she were to win the nomination," he said, "Republicans would end up with a . . . seat in Maryland. If anything, I think Republicans should probably donate to her."