In a speech before about 1,200 people at Ford Field in downtown Detroit, the Michigan native and former governor of Massachusetts outlined his "bold, conservative plan for growth" that includes lowering corporate and individual taxes, reforming Medicare and Social Security, and reducing the federal deficit, The Detroit News said.
Standing at the 35-yard line of the home to the NFL's Detroit Lions, Romney said he has the "best" and "only chance" at victory against President Obama.
"I have the credibility on the economy. The best and the only chance we have to get Obama out of the White House is if we nominate someone whose career isn't politics," he said. "And that's what I represent."
ABC News said supporters sat in folding chairs on the field, surrounded by tens of thousands of empty seats. The Detroit Economic Club said it moved the event from a downtown Detroit hotel because tickets sold out so quickly.
Conservative commentator Laura Ingraham was one of a number of political observers who said the empty seats made for a poor photo op, ABC News reported.
Romney faces a tight race in Michigan against former Sen. Rick Santorum of Pennsylvania but shows a lead in two surveys taken since Wednesday's debate among the four top Republican candidates, the Detroit Free Press said.
Romney's economic plan, previewed Wednesday in Arizona, would lower corporate tax rates from 35 percent to 25 percent, cut individual income taxes by 20 percent, eliminate the 15 percent capital gains tax for those households making less than $200,000 and reduce the top corporate tax to 28 percent.
He said he wants to raise the retirement age for younger Americans and supports a Medicare voucher system that would allow future retirees to purchase private insurance or use the traditional Medicare program.