The failed Republican presidential candidate, who embraced more conservative views as he burnished his GOP credentials nationally, gets mixed reviews among New Yorkers, who are overwhelmingly Democratic.
On Sunday, Giuliani returned to areas of Brooklyn and Queens where he is still well-liked to stump for the billionaire incumbent. Bloomberg, a former Republican, is not registered with any party but is running on the Republican and Independent Party lines.
At a breakfast with Orthodox Jewish leaders in Brooklyn, Giuliani got a standing ovation as he entered the room filled with people who called him a "dear friend'' and recalled his ties with Jewish New Yorkers.
The former mayor, who is also considering a run for governor next year, gave a speech praising Bloomberg for doing "everything he possibly can to keep this city safe.''
Giuliani launched his 2008 presidential bid after honing an image as a counter-terror leader after the Sept. 11 attack, which came in his final months as mayor. He based much of his campaign on the idea that America remains in danger.
Giuliani struck a familiar tone on the campaign trail Sunday, saying the city is still "at great risk.''
And without mentioning Bloomberg's Democratic opponent, William Thompson Jr., Giuliani warned that crime in New York City, which declined under him and has continued to fall under Bloomberg, is also a gamble.
"This city could very easily be taken back in a very different direction,'' Giuliani said. "It could very easily be taken back to the way it was with the wrong political leadership.''
Thompson's campaign released a viral video Friday that sought to highlight Bloomberg's relationships with Republicans, including former President George W. Bush, who was extremely unpopular in New York City.
The montage included clips of Bloomberg praising Giuliani and Bush, along with footage from Giuliani's speech at the Republican National Convention last year where he mocked Barack Obama.
In Brooklyn on Sunday, Bloomberg had even more warm words for his predecessor.
"We all benefited from his leadership, and our city is a much better place because of it,'' he said.
Giuliani is prominently featured on mailings to Republican areas of the city.
A mailer sent to Staten Island voters, headed "Leaders We Trust,'' features a quote from Giuliani and emphasizes law enforcement issues.
"If you want the progress to continue, vote and re-elect Mike Bloomberg,'' Giuliani is quoted as saying.
Campaigning with a Republican star like Giuliani could also help Bloomberg mend relationships with Republicans who were upset with him for abandoning the party as he tested the waters for a 2008 presidential run, and more recently, over his lack of interest in GOP candidates sharing the ticket with him this year.
Bloomberg said recently that he didn't even know there were Republicans running in some races, and a few days later he said "they have no chance whatsoever.''