LaFerla, 63, enters the race after primary winner Wendy Rosen withdrew this week amid allegations that she was registered and had voted in Maryland and Florida. Her name will remain on the Nov. 6 ballot because the deadline to remove it has passed.
"We're ready to fight for every vote in all 12 counties," he said in a statement.
The Maryland Democratic Party made no official announcement Friday.
"The party is working through our endorsement process in the 1st District and will make an announcement at its conclusion," spokesman Matthew Verghese said. "John LaFerla has been a longstanding grass-roots leader of the Maryland Democratic Party and would be a strong candidate against Andy Harris."
Verghese said the party hoped to announce its support for a candidate "in the coming days."
"We are trying to move forward both expeditiously and prudently," he said.
LaFerla had been seen as the favorite to win the party's support this week after losing the primary by just 57 of the more than 25,000 votes cast. He spoke earlier this week of his willingness, if asked, to "step up to the plate and take that responsibility to kind of salvage what's left of this campaign."
"I think that somebody has to give it a try," he said. "I'm willing to do it because I think it's the right thing to do."
Campaign manager Erik Gulbrandsen said staff members were in place across the district and were "moving ahead in hyper-overdrive" to make LaFerla the next representative of "the people, not the 1 percent."
Messages left for the Harris campaign Friday afternoon were not returned. Even before Rosen withdrew, Harris, a 55-year-old physician from Cockeysville, was favored to win re-election to a second term. The 1st District became more Republican last year when lawmakers in Annapolis redrew the state's political map.
Rosen withdrew from the race Monday after she was confronted with the allegations by the Maryland Democratic Party.
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.
Rosen told The Baltimore Sun that she had registered to vote in Florida, where she owns property, in order to support a friend running for the St. Petersburg City Council.
Asked if she had voted in both Maryland and Florida in the same elections, she said she did not remember. Asked if she had voted twice in the 2008 presidential primaries, when Barack Obama and Hillary Clinton competed for the Democratic nomination, she declined to comment "due to possible litigation."
The Maryland Democratic Party referred the matter to state prosecutors; elections officials in Florida said they would do the same there.