Maryland Democratic Party Chairwoman Yvette Lewis wrote a letter on Monday alerting the state attorney general and a state prosecutor to the issue.
"The Maryland Democratic Party has discovered that Ms. Rosen has been registered to vote in both Florida and Maryland since at least 2006; that she in fact voted in the 2006 general election both in Florida and Maryland; and that she voted in the presidential preference primaries held in both Florida and Maryland in 2008," Lewis wrote.
"The Maryland Democratic Party strongly believes in upholding and expanding the right to vote but, at the same time, believes there should be zero tolerance for voter fraud of any kind," she continued.
Reached by TPM at her home in Maryland on Friday, Rosen said she was informed about the discovery in a conference call.
"I don't know where this is heading, and so I am reluctant to make comment on it for legal reasons," Rosen said. But she did confirm she owned a home in Florida and has a friend who was running for office there. She didn't recall whether she voted in-person or via absentee ballot.
"I don't even remember. I usually don't use an absentee ballot," Rosen said. "I'm very concerned about the possibility of litigation, so because this could be an open legal issue I do not want to make a comment on it."
Rosen told the Baltimore Sun on Monday that she registered to vote in Florida to support a "very close friend" running for city council in St. Petersburg.
Rosen was running in Maryland's 1st Congressional District, which covers the Eastern Shore and parts of Anne Arundel, Baltimore and Harford counties.
Maryland law allows the party to designate an alternative nominee after a vote from the Democratic central committees in the district, a process they hope to complete by Sept. 26.
Late update: Harris spokeswoman Kathy Szeliga issued this statement on Rosen's withdraw from the race:
Andy Harris will continue to work hard throughout the district offering solutions to restore our economy and our nation. This development must not delay our troops serving oversees from getting their ballots and exercising their right to vote in such an important Presidential Election.