Romney was asked by a reporter during a stop in a Des Moines suburb whether he would sign the Republican plan if he were president. But the former Massachusetts governor declined to answer.
"That's the kind of speculation that is getting the cart ahead of the horse," he said.
Romney emphasized that he supports the goals of the Republican plan, offered by House Budget Committee Chairman Paul Ryan, but that he would offer his own proposal for reducing spending and cutting the federal deficit.
One of the most controversial aspects of Ryan's plan has been its call to replace Medicare's fee-for-service system with government vouchers.
Earlier, Republican presidential candidate Tim Pawlenty came under pressure after he initially declined to fully support the Ryan plan. He later said he would sign the plan into law if he were president. Fellow GOP presidential hopeful Newt Gingrich faced a backlash earlier after publicly criticizing Ryan's Medicare proposal.
"If we don't make any changes, then Medicare won't be there for the next generation and that's unacceptable to me. So I appreciate what Paul Ryan has done," Romney told reporters in Ankeny after visiting an agricultural software company. "I'm going to have my own plan."
Romney was making his first visit to Iowa since formally beginning to explore a second bid for president. He plans to announce his 2012 candidacy next week in New Hampshire. He finished second in the state's leadoff presidential caucuses last time and is expected to wage a more limited campaign.
His carefully planned day hit a snag during an appearance in Des Moines. A fire alarm triggered by burnt microwave popcorn, forced the evacuation of a museum during a question-and-answer session that followed Romney's speech.
That sent Romney and the roughly 200 people attending the event onto the street outside.
Romney, who served one term as Massachusetts governor, is stressing as he approaches the 2012 bid his business and private sector background as an investment company executive and CEO of the 2002 winter Olympics.
Aides said Romney did not develop this niche in 2008 and at commented too often on less familiar cultural issues, which hurt him in socially conservative Iowa. Romney had supported abortion rights, gay rights and gun control as governor.
Romney returned to Iowa focused squarely on the economy.
"What I know and what I've spent my life doing is particularly relevant right now," he told the 200 people at the museum event, including Republican activists and past supporters.
In Des Moines, he set himself up as the best alternative to President Barack Obama on economic issues. He said the Democrat's legislative agenda, including the 2010 health care law, had sown uncertainty at a time investors needed encouragement.
"It's time to have a president how to create jobs, because he's had one, and knows how the economy works for the American people."
Romney, who leads in most national polls of GOP preference, does not need the lift to his profile he sought in Iowa in 2008, aides say.
He planned to meet privately Friday in eastern Iowa with supporters of his 2008 before hosting a picnic in Cedar Rapids for Republicans in Linn and Johnson counties, which he carried in the caucuses last time.
But he stopped short Friday of saying whether he would campaign for the state GOP straw poll. Romney won the Iowa straw poll in August 2007, after spending heavily to organize for the event.
"We're going to do what we think is best calculated for me to become the nominee and for me to win the White House," Romney said.
Des Moines Republican Dixie Belluchi-Watters, who backed Romney in the 2008 caucuses, said some Iowa Republicans would turn away from him if he skips the straw poll.
"It doesn't matter to me," she said. "But in the long run, it could hurt him here. He has an opportunity here to be the strongest business candidate."