It was unclear, however, whether she could remove her name from the ballot with the election less than two months away. Under state law, a candidate has until 70 days before an election to remove his or her name from the ballot. The deadline for the Nov. 6 election passed on Aug. 28.
Democratic leaders — who raised the allegations, urged Rosen to step aside and notified prosecutors — said they would gather Central Committee members this month to identify a write-in candidate for the district, which includes the Eastern Shore and parts of Harford, Carroll, Cecil and Baltimore counties.
Republicans, meanwhile, said the allegations prove that voter fraud is real and called on Democrats to join the GOP in calling for reforms.
Rosen, 57, a Cockeysville businesswoman and Maryland voter, told The Baltimore Sun that she registered to vote in Florida several years ago in order to support a "very close friend" running for the St. Petersburg City Council and to vote on local issues there.
Rosen said she was able to register in Florida because she owned property there.
Under Maryland law, a voter here may not maintain registration in a second state if it allows the voter to participate in state or federal elections there, according to Jared DeMaris, director of candidacy and campaign finance at the State Board of Elections.
State Democratic Chairwoman Yvette Lewis said an examination of voting records in Maryland and Florida showed that Rosen participated in the 2006 general election and the 2008 primaries in both states.
Maryland and Florida both held gubernatorial and congressional contests in 2006 and presidential primaries in 2008, when Barack Obama and Hillary Clinton competed for the Democratic nomination.
Asked by The Sun on Monday if she had voted in both states in the same elections, Rosen said she did not remember how she voted. Asked if she had voted twice in the 2008 presidential primaries, she declined to comment "due to possible litigation."
Lewis referred the allegations Monday to Attorney General Douglas F. Gansler and State Prosecutor Emmitt C. Davitt.
"We believe that this is a clear violation of Maryland law and urge the appropriate office to conduct a full investigation," she 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."
DeMaris, the state elections official, said there are narrow circumstances under which a Maryland voter may register legally in two places.
Some municipalities maintain separate voting rolls for local elections. These allow property owners — even those that live and vote elsewhere — to register and participate in elections for local offices and ballot questions.
Thus, DeMaris said, a resident of Baltimore who owned a vacation home in Ocean City could legally vote in local elections there, too.
DeMaris said he knows of no law against a Maryland voter participating in local elections in another state. But an out-of-state registration that permitted participation in state and federal elections would be illegal, he said.
A spokesman for Maryland Democrats said party leaders were told of Rosen's dual registration by someone within the party on Friday. After verifying the details over the weekend, spokesman Matthew Verghese said, the leaders confronted Rosen on Monday.
Rosen was seen as the underdog to Harris, a 55-year-old physician from Baltimore County who is serving his first term in Congress. The district that has grown more solidly Republican since Harris ousted Democratic Rep. Frank Kratovil in 2010.
Still, she mounted a highly visible effort to promote her candidacy in July at the annual J. Millard Tawes crab feast in Crisfield.
At the time, she told a reporter she believed Harris was vulnerable to a challenge because of unpopularity among voters.
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