On Monday, Koo headed to the Queens County Board of Elections on Queens Boulevard in Kew Gardens and officially changed his party designation from Republican to Democrat, leaving only four Republicans in the City Council.
He was joined by Congressman Joseph Crowley, the head of the Queens County Democratic Party, and several of his – now – fellow Democratic elected officials.
"This has been a homecoming in many respects for the Flushing community," said Crowley. "I think this keeps in line with the tradition of their representation and is representative of the diversity not only of Flushing, but of Queens County."
Officially, Koo, a successful Asian-America businessman, said his decision was based on ideology, and that he found himself increasingly at odds with his Republican colleagues on important issues, most notably immigration.
"I am a first-generation immigrant, so I understand how hard it is to be a newcomer in this community and I always sympathize with immigrant issues," said Koo. "It's important for them to have a way to stay here."
But those familiar with Koo said he was disappointed by the lack of support that he was receiving from the Queens County Republican Party, as well as being shut out of major decisions, such as endorsements and the push to create another Asian majority district for the State Senate in northeast Queens, that led to his decision.
However, Koo refused to criticize the Republican Party on Monday, only addressing it briefly.
"The Republican party, of course they're sad that I'm leaving, but there's not much they can do," he said.
While Queens GOP leadership was quoted in published reports that they were surprised by the decision, there were hints that Koo would jump ship.
About six months ago, he suggested that he was thinking about changing his party affiliation, and in the special election to replace Congressman Anthony Weiner, Koo endorsed Democrat David Weprin over fellow Republican Bob Turner. Turner would eventually win.
According to a source, Koo was also concerned about his ability to win re-election if he remained a Republican in a race for a seat the Democrats would aggresivly push to win back.
Robert Hornak, a spokesperson for the Queens County Republican Party, wished Koo the best.
"We were very disappointed to see him leave the Republican Party, and thought highly of him as a Republican legislator," he said. "We know he will still admirably serve the community, and look forward to having a relationship with him in the future."
Also on hand for Koo's announcement was Comptroller John Liu, who despite a long relationship with Koo, a prominent Flushing businessman, endorsed his Democratic challenger in 2009.
"We're happy that he is back home because the issues that he's talked about on the campaign trail, and what he's done in the City Council, is really very much in line with our Queens delegation," said Liu. "So it's only natural that Peter Koo is about to be a Democrat."
Liu also praised Koo's history of community service in Flushing.
"Peter has been a great friend to the Flushing and Queens community long before he sought any kind of office,” he said. “He was such an entrepreneur and successful business person, but somebody who never forgot about the community."