"Mayor Bloomberg is someone who represents the same approach to public service I believe in - let's solve the problem, take ideas from both sides, and resolve issues in a pragmatic way," Sestak said Sunday in a brief interview. "It's an honor."
Bloomberg, a former Wall Street trader who founded the financial media and information empire that bears his name, began his third term as mayor of the nation's largest city in January. Elected first as a Republican, he left the party halfway through his second term.
He briefly considered an independent run for president in the 2008 election.
"The mayor is supporting Joe Sestak because he's an independent thinker, he's intelligent, and he has executive experience," Jason Post, a Bloomberg spokesman, said. He has endorsed a handful of candidates who match his values, Post said.
Bloomberg reached out with an offer of support several weeks ago, Sestak campaign officials said. They said that the mayor indicated he also liked Sestak's emphasis on tax breaks and lending initiatives aimed at small businesses.
New York's economic-development effort is geared toward small business, with business-incubator, seed-funding, and loan programs, as well as workforce training, according to the city administration's website.
Bloomberg's visit could trigger questions on one of the most controversial issues in politics in recent days: Muslim plans to build a community center and mosque near ground zero in Lower Manhattan.
Republicans have been pounding President Obama for his defense of the right of Muslims to build there.
Obama spoke up Friday at a dinner for Muslims at the White House, and then recalibrated his remarks Saturday to add that he did not intend to endorse the "wisdom" of placing a mosque near ground zero, only the right to do so.
The issue could be sticky for Sestak, who was attacked in TV ads in July by a conservative group for speaking before the advocacy group Council on American-Islamic Relations in 2007. CAIR's opponents have branded it as a front for the extremist group Hamas.
Sestak's campaign tried unsuccessfully to get the ad removed from the air, saying it was inaccurate because it accused him of raising money for CAIR. He said he spoke only during a portion of the event that he insisted not be devoted to fund-raising. (CAIR started its fund-raiser after he was done speaking.)
Bloomberg, who is Jewish, has vocally defended the mosque project.
"Should government attempt to deny private citizens the right to build a house of worship on private property based on their particular religion?" Bloomberg said in a speech Aug. 3. "That may happen in other countries, but we should never allow it to happen here," he said.
Contact staff writer Thomas Fitzgerald at 215-854-2718 or firstname.lastname@example.org.