LIVONIA, Mich. – On the eve of a Michigan showdown, Mitt Romney and Rick Santorum swapped insults Monday in a struggle for the Republican presidential nomination growing so long and heated that party officials fretted openly it could harm prospects for winning the White House this fall.

On this day, the subject was their competing plans for the economy.

“Sen. Santorum is a nice guy, but he’s never had a job in the private sector,” Romney said as he and his closest rival charged across the state in a final day of pre-primary campaigning.

Santorum said Romney’s tax cut plans mirror the rhetoric of Occupy Wall Street and include “just more Obama-style class warfare.”

The Santorum campaign sponsored computerized phone calls urging Michigan Democrats to vote against Romney in the state’s Republican primary, which is allowed if they declare themselves Republicans for the purpose of voting. Romney called the effort a “dirty trick” in a Fox News interview Monday night, but Santorum defended the “robocall” as positive and told the network that the calls were part of an effort to attract Democratic voters he would need in a general election.

The ubiquitous polls showed a close race in Michigan, where Romney was born and won a primary in his first bid for the White House four years ago. Santorum surged unexpectedly into contention two weeks ago, benefiting from caucus victories in Minnesota and Colorado and stressing unflinching conservative views on social issues. No matter the winner, the two men stand to split the 30 delegates at stake.

By contrast, Romney is favored to capture Arizona and all 29 delegates in the night’s other primary. There, the campaigning has been scarce and the television commercials ever scarcer, sure signs that Romney’s rivals have scant hope of an upset.

Neither of the other two contenders, Newt Gingrich or Ron Paul, has made much of an effort in either Michigan or Arizona.

But Gingrich, the former House speaker, said Santorum could face a far different race if he loses to Romney in Michigan.

“He’s had two weeks of being the alternative (to Romney). The fact is, I think there are profound reasons that Rick lost the Senate race by the largest margin in Pennsylvania history in 2006, and I think it’s very hard for him to carry that all the way to the general,” Gingrich said, eager for a comeback of his own.

Though it’s an important prize, Michigan is also prelude to Washington caucuses on Saturday, with 40 delegates at stake, and especially Super Tuesday on March 6, when 10 primaries and caucuses are on the ballot with 419 delegates.

Romney currently has 123 delegates to 72 for Santorum, 32 for Gingrich and 19 for Paul in the Associated Press count, with 1,144 required to win the party nomination this summer at the Republican National Convention in Tampa, Fla.

Fifty of Romney’s delegates were the result of a winner-take-all primary in Florida, meaning that Santorum is nearly even with him elsewhere. After Arizona, nearly all the remaining states will split their delegates based on the popular vote, making it harder for any candidate to shut out his rivals.

As a result, Republican governors attending the National Governors’ Association conference in Washington over the weekend expressed concern about the impact of a long race on their party’s chances for defeating President Obama.

“I don’t know anybody who thinks if you started out to design a good process to pick a president you’d choose exactly what we have now,” said Indiana Gov. Mitch Daniels.

Gov. Paul LePage of Maine, a Republican elected with tea party support in 2010, said, “If they continue to beat each other up, then maybe we should get somebody unknown to go against Obama. They’re damaging themselves.” “It’s like a marital battle,” he said. “Somebody’s got to apologize.”

There seemed no chance of that happening in Michigan, where Romney and Santorum battled at close quarters for supremacy in the first of the nation’s big industrial states to hold a primary.

Both men have struggled to find a campaign balance between economic issues that affect most if not all voters in a state of 9 percent unemployment, and social issues that are typically of greater concern to conservative voters.

Romney has twice appeared before tea party audiences in the past several days, hoping to forge a bond with the insurgents who have helped reshape the Republican party in the state.

For his part, Santorum went before the Detroit Economic Club – hardly a natural fit for him – in pursuit of a victory that could upend the race.