DES MOINES, Iowa — President Barack Obama delivered his harshest rebuttal yet to rival Mitt Romney on Thursday, dismissing his challenger’s claims as “a cowpie of distortions” while seeking to rekindle the all-but-faded Iowa magic that launched him in 2008. Escalating his criticism of Romney’s background as a venture capitalist, Obama said it wasn’t adequate preparation for the presidency.

“There may be value for that kind of experience, but it’s not in the White House,” Obama said.

The speech, to a cheering Iowa crowd of about 2,500 at the Iowa State Fairgrounds, represented a new intensity for Obama’s campaign as Romney begins to hit his stride carrying the Republican standard. It came as Iowa, soured by the direction of the nation and its economy, has drifted away from Obama since his 2008 caucus victory over Hillary Rodham Clinton made him the Democratic front-runner.

While Obama carried the state in the general election by a comfortable margin that year, polls this year have shown voters narrowly preferring Romney, who plans to wage his own major effort in Iowa.

Trying to cast Romney as out of touch with working-class voters, Obama declared that hard work hasn’t led to higher incomes. “Higher profits haven’t led to better jobs,” he said. “And you can’t solve that problem if you can’t even see that it’s a problem.”

Obama pointedly chose the same turf where Romney, the presumptive Republican nominee, once declared that corporations are people. Obama said Romney would roll back regulations and return to policies that he said helped create the recession and would increase government deficits.

Reacting to Romney’s charge last week that Obama had created “a prairie fire of debt,” the president countered that Romney’s tax plan is “like trying to put out a prairie fire with some gasoline.”

In a statement issued after the speech, Romney spokesman Ryan Williams said: “A president who broke his promise to cut the deficit in half by the end of his first term has no standing when it comes to fiscal responsibility.”

Earlier, in blue-collar Newton, Iowa, once the prosperous headquarters of Maytag appliances, Obama visited a wind-turbine plant to push his alternative energy agenda and delivered a message that could as well have applied to all of Iowa. “Yeah, we’re facing tough times, but we’re getting through them, we’re getting though them together,” he said.

While offering only six of the 270 electoral votes needed to win the presidency, how Iowa voters ultimately judge Obama is expected to be an important factor in the race.

“Last time it was a lot more exciting. It was a new thing,” said Nancy Bobo, a Des Moines Obama volunteer and one of his earliest Iowa backers in 2008. “Today, we’re all just very serious.”

Romney, a former Massachusetts governor, has made the struggling economy the centerpiece of his campaign. But Obama can point to comparatively low 5.1 percent unemployment in Iowa, where stable financial services and strong agriculture sectors buoyed the economy while manufacturing has struggled to rebound.

Romney had made the comment about corporations as he argued against raising taxes as a way of shoring up Social Security and Medicare. Members of the audience interrupted, calling for increased taxes on corporations, and Romney responded: “Corporations are people, my friend. … Everything corporations earn ultimately goes to people.”

The comment has been used by opponents to characterize Romney, a former private equity firm executive, as more comfortable in the boardroom than the shop floor.

Obama’s campaign has emphasized episodes in which Romney’s former firm closed plants and laid off workers, and has aired a stinging TV ad on the subject in Iowa, Colorado, Ohio, Pennsylvania and Virginia.

In his fairgrounds’ speech Thursday, Obama said private-equity firms can sometimes create jobs. “But when maximizing short-term gains for your investors rather than building companies that last is your goal,” he added, “then sometimes it goes the other way. Workers get laid off. Benefits disappear. Pensions are cut. Factories go dark.”

Obama himself has struggled to attract blue-collar voters, keys to winning struggling swing working-class regions such as southeast Ohio, western Pennsylvania and rural Iowa. Newton is the seat of Jasper County, Iowa, where unemployment was 7.1 percent in April, higher than Iowa’s average but down sharply from last winter.

While Iowa is known for its first-in-the-nation caucuses, it also is a coveted general election state, despite its small electoral total. Democrat Al Gore carried the state by less than a percentage point in 2000, followed by Republican George W. Bush’s 2-point victory in 2004.

Obama has already spent more than $2.6 million on advertising, a pace as aggressive as in any other battleground state. He’s been a regular visitor, and was making his second trip in a month.

Yet the president’s approval rating here has been stuck below 50 percent for over two years, softened in part by criticism from Republicans campaigning for Iowa’s leadoff caucuses.

Polls show Iowans also have become increasingly bothered by federal spending, an issue Romney stoked in Des Moines last week in a visit where he promised to shrink the deficit.

Iowans, many of whom met Obama in the 2008 campaign, also are disappointed by what they hoped would be a transcendent presidency, said J. Ann Selzer, the longtime director of The Des Moines Register’s Iowa Poll.

“You hear disaffection. You hear them say, ‘This isn’t what I paid for,”’ Selzer said. “The guy they sent there to recast things wasn’t able to do it.”

Privately updating Senate Democrats on Thursday, Obama campaign manager Jim Messina said the president has several possible paths to collecting the 270 electoral votes he needs for victory in November. But he noted that Romney, the Republican Party and allied super PACs are likely to have a great deal of money to spend. Obama vastly outspent Republican John McCain in winning the White House four years ago, an advantage that Democrats appear unlikely to command in 2012.

Romney senses the opening. He too has cultivated an Iowa network. Indeed, he campaigned aggressively for the 2008 caucuses during his narrowly losing bid for the state’s delegates. Romney and the Republican National Committee have hired state directors and are hiring staff to run a dozen or more offices planned for Iowa.

And his campaign has begun running television ads in Iowa.