CIA director Mike Pompeo had scrapped a planned appearance at the Nantucket event over the title for Ms Manning, calling her an "American traitor".
It was while in prison that Manning received gender reassignment surgery.
Speaking at the New Yorker Festival in October, Manning said she fears for her safety but does not plan to lay low.
The Washington Post said Mr Cardin, who is Maryland's senior senator, is not considered especially vulnerable to a challenge from within the state.
Manning was convicted in 2013 for stealing sensitive government documents and videos, including information about the war efforts in Iraq and Afghanistan, State Department cables, information about prisoners in Guantanamo Bay and leaking the information to WikiLeaks. After the sentencing, Manning, who was then known as Bradley, identified as transgender and changed her first name to Chelsea. In January 2017, former President Barack Obama commuted her sentence to nearly seven years of confinement dating from her arrest on May 27, 2010.
Cardin spokeswoman Sue Walitsky did not directly address Manning's candidacy or anyone else's.
Manning, originally from Oklahoma, moved to Maryland after her release.
"Our only statement on the record is 'No statement, '" Manning said. Now Manning is set to make history herself with an eye for the Senate seat. I've been in the headlines. When reports broke last week that Immigration and Customs Enforcement agents were raiding 7-11 stores, Manning called the agency "the new Gestapo". In Jan. 2017, she authored an editorial in The Guardian in which she praised Obama's leadership and blasted his critics as stubborn and narrow-minded.
Cardin is also yet to file. His campaign war chest already boasts funds upwards of $2 million.