About 90 people paid $1,000 each for a breakfast buffet of cold cereal, fresh fruit, sweet rolls and tidbits from Bush about why Romney should beat Democratic incumbent Carl Levin.
"Ronna may be behind ... but I'll guarantee you it's going to close and close fast," Bush said. "We saw today why - (Ronna is) aggressive, bright, able. Those are character traits that I think people of Michigan identify (with)."
Michigan voters crave the values Romney represents, including better schools, less crime and family-friendly policies, he said.
"And I believe that she can attract the interest of the business community because she knows that a job is far better than some failed government program," he said.
Bush called Romney's race one of his top political priorities after only the careers of his sons George and Jeb and the presidential campaign of Bob Dole.
And he urged Republicans to ignore polls showing her 20 points behind Levin and to work hard.
"My heart is in this race. I don't give one tinker's damn what the numbers are today. That is not what counts," he said.
Bush called his appearance a chance "to pay back somebody who gave far more than she got."
Romney was Michigan co-chair for Bush's 1988 and 1992 presidential campaigns.
"I'm confident, I really believe that people in this state want to have a woman senator. It's not going to be done just on a gender thing. ... I think it's time."
Romney recalled that she was the first GOP national committee member to endorse Bush's presidential candidacy in 1988 during one of his visits to Detroit. The then-vice president was so appreciative he turned his motorcade around and came back to thank her when he heard about it, she recalled.
Romney said Bush's appointment of her as chair of his Commission on White House Fellowships and his willingness to meet with her when she visited Washington made him special to her.
"How wonderful it is to be able to say George Bush not only was my president, but he was also my friend and somebody I could always count on and can still always count on. He is the best of what we have to offer today in America for leadership," she said.
Afterward, Romney said Bush's appearance helped to garner public attention for her race and to raise money to offset Levin's anticipated edge in funding.
She said early polls are meaningless and predicted it would soon tighten as traditional Republican voters come back into the fold.
Levin campaign spokesperson Steve Serkaian said Bush was a fitting backer for Romney.
"It's not surprising Ronna Romney asked George Bush to campaign for her, as Ronna Romney's extreme economic policies would turn the clock back to the failed economic policies of the Bush years," he said.