Giuliani spoke before a few hundred cheering Coleman supporters at a Bloomington hotel.
"The people of Minnesota should not be asked to provide on-the-job training for somebody who has no experience and no background in government," Giuliani said.
The times call for senators with experience and a government record, Giuliani said at a rally for Coleman at a suburban hotel. Giuliani never spoke the name of Coleman's Democratic opponent, Al Franken, but his comments were aimed at Franken's background as a comedian and entertainer.
"It's not a game, it's not entertainment, it's not a joke. Government is for real," Giuliani said.
He praised Coleman, underscoring Coleman's experience as a mayor and a Senator, and he compared Coleman to his DFL challenger without mentioning Al Franken by name.
"When he gets up in the United States Senate to speak nobody's going to be laughing, nobody's going to be giggling," Giuliani said. "Nobody's going to say, 'How did this guy get here?' You want that from Minnesota? You want the reaction be be how did he get here? What did they do?"
Coleman and Giuliani each noted parallels in their backgrounds. They are both former government prosecutors and urban mayors. They stressed an ethic they said is taught by mayoral leadership, that the heart of politics is accomplishing things for constituents.
"It's about results. It's not about rhetoric," Coleman said.
Colleen Murray, a spokeswoman for Franken's campaign, said Coleman "has a lot of experience fighting on behalf of the special interests and standing up for the failed policies of George Bush. That's not the type of experience Minnesota's middle class needs or deserves."
Coleman finds himself a few points behind Franken in several recent polls; as he has recently, Coleman touted his decision to stop negative TV ads by his campaign, and said he would like to be part of reducing partisan tension in Washington.
"There are issues in this country that are bigger than what one party can solve," Coleman said.
Giuliani and Coleman defended Giuliani's recent recording of automated "robocalls" that are critical of Democratic presidential candidate Barack Obama's lack of support for mandatory minimum prison sentences for some crimes. The calls have not been placed in Minnesota, but have turned up in other states including Wisconsin. Coleman last week condemned McCain's robocalls, but said Thursday that the contrast Giuliani tries to draw in his calls is a fair one.
"It wasn't a personal attack, it wasn't negative campaigning at all," Giuliani said. "I pointed out that Barack Obama didn't support mandatory minimum sentences. I've worked very hard for mandatory minimum sentences. That's just his record."
Giuliani will campaign with Coleman in southern Minnesota on Friday.
(The Associated Press contributed to this report.)