WASHINGTON (CNN) -- Former New York City Mayor Rudy Giuliani, who was once a registered Democrat and is now seeking the GOP nomination for president, defended his political evolution during a speech at the Hoover Institute Monday.

Quoting former British Prime Minister Winston Churchill, Giuliani said, "If you're not a liberal when you're 20, you have no heart. But if you're not a conservative by the time you're 40, you have no brain."

Giuliani citied the Democrats' position on the economy as the chief reason why he switched his affiliation, arguing that Democrats do not understand that "we are essentially a private economy."

"Maybe way back then, in the era of Kennedy, they understood it, too," Giuliani said. "And they did, but they have kind of lost that -- one of the reasons I used to be a Democrat and am now a Republican.