At a rally at the Airtel Plaza Hotel, Giuliani said Whitman will cut spending and lower taxes in California.
"This is the right person at the right time for the kind of challenges California faces," Giuliani told Whitman's supporters. "Sometimes I think California may be more complicated than America."
He also criticized President Barack Obama's economic policy and said California needs an executive in the governor's office.
In the 2008 presidential race, Whitman was an economic adviser to G.O.P. nominee, Senator John McCain.
Whitman's rival, Democrat and current state attorney general, Jerry Brown, did not hold any public campaign events Sunday.
In Pasadena, Senator Barbara Boxer picked up a key endorsement in her re-election campaign.
The Veterans of Foreign Wars pledged their full support to the Democratic senator.
She promised to fight for veterans' rights, but she also vowed to help the economy by bringing jobs back to the U.S.
Boxer's opponent, Republican Carly Fiorina, talked about jobs at a campaign event in Century City.
She said some blame Boxer for California's bleak economic picture.
"Jobs are leaving California because of her policies," Fiorina said. "Manufacturers say Boxers policies have been the worst ever in terms of destroying jobs."
Fiorina addressed the Mideast question, saying Israel should not have to stop building new West Bank settlements for peace talks to continue.