Many upcoming primaries are open to members of all parties. That favors McCain.
For anyone doing the political math, Michael Martin is the unknown variable.
The bespectacled computer salesman is a Democrat, but come Tuesday, he might just vote for Arizona Sen. John McCain in Michigan's Republican presidential primary.
Texas Gov. George W. Bush - with his strong Republican support - sees Democratic and independent voters like Mr. Martin as meddling outsiders. But for Senator McCain, these "crossover voters" have become his most crucial constituency - the key to his crushing victory in New Hampshire and vital to his future success.
Considering that 10 of the 17 states holding GOP primaries between now and March 7 allow crossover voters, such voters will play a major role in determining the Republican nominee. If they turn out in big numbers, they will help McCain stay competitive, and perhaps even win.
From now until "Titanic Tuesday" on March 7, "independents are the force to be reckoned with," says pollster Del Ali of Research 2000.
If Governor Bush can halt the march of independents toward McCain, he'll deal a serious blow to the Arizona upstart's prospects of snatching the nomination.
Observers say Bush's unveiling this week of a retooled campaign-finance plan in South Carolina - which holds its primary on Saturday - was aimed at doing just that. The proposal, combined with other efforts, appears to be having an effect. Bush has edged up slightly in recent Palmetto State polls.
But here in Michigan - the state, along with Arizona, where the national spotlight will shift immediately after South Carolina - he's behind, despite strong backing from popular Republican Gov. John Engler.
McCain had been trailing Bush here, but after winning New Hampshire, he jumped to a 9-point lead - 43 percent to 34 percent - according to a Detroit News poll this week.
For Martin, who's catching a quick fast-food dinner at McDonald's after work, McCain is the true reformer and maverick.
"Anyone who can rattle the established status quo is definitely interesting to me," he says. "Do I worry sometimes about having him in the Oval Office - that he's a loose cannon? Yeah, I do."
But in the end, it's probably worth it, says the 40-something with a salt-and-pepper beard and three children, who's also pursuing a technology-related PhD.
He's symbolic of the county he lives in - Oakland County, a veritable gold mine of independents in suburban Detroit. This fast-growing area is bursting with mostly white voters who are typically well-educated, wealthy, conservative on money matters, and more moderate on social issues.