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 that it could harm prospects for winning the White House this fall.

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

"Senator 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 campaigning.

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

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 in 2008.

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.

Romney is favored to capture Arizona and all 29 delegates in today'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.

The other two contenders, Newt Gingrich and Ron Paul, haven't made much effort in either Michigan or Arizona.

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

Though it's an important prize, Michigan is also a prelude to Washington's 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 has 123 delegates to 72 for Santorum, 32 for Gingrich and 19 for Paul in the AP count, with 1,144 required to win the 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 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.