But on Monday night, he unleashed a stinging barrage against her character in the most energetic speech of the Republican National Convention's opening night.
Sixteen years after he'd been forced to pass on his Senate campaign amid a divorce and a bout with cancer, Giuliani pounded Clinton with everything he had.
In a speech introducing Donald and Melania Trump, Giuliani stuck to the day's themes: attacking Clinton over Benghazi and immigration.
"Who would trust Hillary Clinton to protect them? I wouldn't. Would you?" he said.
"Hillary Clinton's experience is the basis for her campaign," Giuliani said. "Hillary Clinton's experience is exactly the reason she should not be president of the United States. There's no next election -- this is it. No more time to repeat our mistakes of the Clinton-Obama years. Washington needs a complete turnaround."
Giuliani, the former New York City mayor, and Clinton, once New York's senator, have traveled in similar political circles for years -- first in the 2000 Senate race, and then in the 2008 presidential primary, when both lost their parties' contests -- without ever facing off.
Giuliani gestured vigorously -- at one point referring to Trump helping people "from the top to the bottom" while pointing just the opposite directions, low and then high -- while the crowd roared.
Most of the delegates were on their feet, including New Jersey Gov. Chris Christie, who was at the back of his state's delegation.
Giuliani also offered a strong testimonial for Trump's personal qualities, telling the crowd he'd known Trump for 30 years and had watched him anonymously contribute to help police and firefighters in need.
"He has created and accomplished great things in my city and all over the world, but beyond that this is a man with a big heart," Giuliani said. "Every time New York suffered a tragedy, Donald Trump was there to help."
Playing to the day's law-and-order theme, Giuliani also sought to rally support for police officers.
"We know the risk you're taking and we say thank you to every police officer and law enforcement officer out tonight," he said. "When they come to save your life, they don't ask if you're black or white -- they just come to save you."