Democratic voters in Michigan may be a key element in picking the Republican presidential nominee if, as expected, they turn out to be the deciding factor in determining the result of the Michigan primary today.

The race between Senator McCain and Mitt Romney in the Wolverine State is so close that it may be decided by Democratic and independent voters who choose to vote in the Republican primary.

Such an outcome is made more likely by the fact that the Democratic presidential contenders have decided not to campaign in Michigan, after state Democrats brought forward the date of the primary, contrary to party rules. A number of opinion surveys put Messrs. McCain and Romney in a dead heat, with Michael Huckabee trailing in third place. The two most recent polls offer conflicting opinions about who exactly is in the lead.

A poll completed on Sunday by Zogby for C-Span and Reuters suggests that Mr. McCain is leading by a nose with 27%, with Mr. Romney three points behind at 24%, and Mr. Huckabee third with 15%.

Polling by Mitchell Research taken over a similar period gives Mr. Romney a slight edge at 29%, with Mr. McCain two points behind at 27%, and Mr. Huckabee the back marker at 12%. The margin of error of both polls means the front-runners are statistically neck and neck.

The polls show that Democratic and independent voters could be pivotal in deciding who wins tonight. As the result of each primary drastically alters the next, the Michigan verdict will alter the national contest significantly.

Democratic and independent voters in Michigan could therefore end up having a disproportionate and, Republicans might argue, inappropriate influence over who becomes the Republican presidential nominee.

The entrails of the polling show how important the Democrats and independents are in Michigan and that Mr. McCain, rather than Mr. Romney, is benefiting by two to one.

According to pollster John Zogby, independents make up 35% and Democrats 17% of those intending to vote in the Republican primary. American Research Group puts independents at 27% of those voting.

Independents split 33% for Mr. McCain, 18% for Mr. Romney, and 12% for Mr. Huckabee, according to Zogby figures. ARC says Mr. McCain wins independent voters by a wider margin, 43% to 19%, over Mr. Romney, with Mr. Huckabee taking 14%.

Twice as many Democrats intending to vote in the Republican primary back the senator from Arizona, with 35% for Mr. McCain and 17% for Mr. Romney, according to Zogby.

“The premise that Michigan Democrats can play a major part in picking the Republican nominee is not far fetched,” Mr. Zogby told The New York Sun. He said that over each of the last three days of polling he had recorded a consistent trend for Democrats and independents to favor Mr. McCain.

“Eight years ago, McCain went from victories in Michigan and his home state of Arizona, with Virginia, North Dakota, and Washington state along the way, into Super Tuesday,” Mr. Zogby said. “This time around, the result in Michigan can springboard him into South Carolina.”

“Ultimately, two victories in a row, in New Hampshire and Michigan, can put him well on the road to the nomination,” he said.

The president of the polling company Mitchell Interactive, Steve Mitchell, has discerned another trend. While independent and Democratic voters in the Republican primary represent a large number, over time they are being diluted by late-deciding Republicans who are backing Mr. Romney.

“As the undecided voters make up their minds, more are turning to Mitt Romney than to John McCain,” Mr. Mitchell reports. “We have also seen the participation among Republicans increase from 62% [on Saturday] to 75% at the end of phoning [Sunday night]. That means that 75% of the voters taking part in the GOP primary identify themselves as Republicans.”

The campaign in Michigan turned to the state of the economy yesterday, with heated exchanges between the two main Republicans.

“Michigan is a bit like the canary in the mine shaft. What’s hurting Michigan, if left unchecked, will imperil the entire nation’s economy,” Mr. Romney told the Detroit Economic Club before condemning Mr. McCain’s proposals for greener energy.

“The Energy Information Agency estimated that this bill would raise electric rates by as much as 25% and gasoline prices by 68 cents a gallon. And the cost in American jobs: over 300,000,” Mr. Romney said.

One of Mr. McCain’s supporters, Jane Swift — who served as acting Massachusetts governor between 2001 and 2003, when she was succeeded by Mr. Romney — soon dispatched a return salvo.

“Michigan voters shouldn’t be fooled by Mitt Romney’s latest campaign promises of economic development and job growth,” she said in a statement. “They need to look closely at his record in Massachusetts, where manufacturing job growth was third worst in the country and taxes and ‘fees’ increased by over $700 million per year.”

“Mitt Romney has a reputation of saying anything to win. A comparison of his record versus his rhetoric on the economy shows why,” she said.