SOUTHFIELD, Mich. -- It's now a three-way race.

Mitt Romney scored his first major primary victory Tuesday in his native Michigan, and set the stage for a Republican showdown in South Carolina in just four days.

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Romney was the third Republican victor in the first four states to vote in the 2008 primary season, further roiling a volatile nomination fight that lacks a clear favorite.

The former Massachusetts governor defeated John McCain, the Arizona senator who was hoping that independents and Democrats would join Republicans to help him repeat his 2000 triumph here. Mike Huckabee, the former Arkansas governor, trailed in third, and former Tennessee Sen. Fred Thompson was waiting for the top three candidates in South Carolina, already campaigning.

"It's a victory of optimism over Washington-style pessimism," Romney said in an Associated Press interview, echoing his campaign speeches. "Now on to South Carolina, Nevada, Florida."

"I congratulate the governor. I just talked to him on the phone and congratulate him on his victory. Starting tomorrow, we're going to win South Carolina, and we're going to go on and win the nomination," McCain said in his concession speech.

"I congratulate him on that Michigan welcomed their native son with their support," McCain said, but added, "I said we would win in New Hampshire. We will win in South Carolina."

Hillary Clinton won the Democratic competition, an essentially meaningless victory after her party stripped Michigan of its delegates in retribution for trying last year to jump ahead of New Hampshire on this year's primary calendar.

Clinton, Sen. Barack Obama and Sen. John Edwards skipped campaigning in Michigan. She faced competition principally from the "uncommitted" line on the ballot, an option that some supporters of Edwards and Obama advocated to embarrass the former first lady.

The Michigan votes were counted as Clinton, Obama and Edwards focused on a two-hour debate. It was their last encounter before Saturday's Nevada caucuses, the next for-keeps contest in their battle for the party's nomination.

Turnout Low

A limited Democratic ballot combined with a fresh blanket of snow held down the Michigan voter turnout.

Up to six inches of fresh snow in some parts of the state.

Election officials and analysts had estimated roughly 20 percent of Michigan's 7.14 million registered voters would participate in either the Democratic or Republican primaries by the time polls closed at 8 p.m.

But the lack of an active Democratic contest appeared to drive turnout below that, particularly in some of Michigan's largest counties and cities. Many of them were closer to 15 percent as polls were closing.

A record 1.4 million voters participated in a Republican-only Michigan primary in 2000, a race that also drew Democrats and independents. In 2004, about 160,000 voted in the Democratic presidential party caucuses.

Republicans had a full ballot but Democratic primary voters had fewer choices, since Hillary Clinton was the only major candidate on the list. Barack Obama and John Edwards pulled their names from the Democratic ballot after Michigan broke national party rules by moving up its primary date.

An exit poll for The Associated Press and television networks indicated seven in 10 voters Tuesday opted to vote in the Republican primary. Among those who did vote on the Democratic side, more than three in four called themselves Democrats.