And while neither contest will decide its party's nomination, both will provide clues.
In the GOP race, Florida's primary on Jan. 29 will be the first time all the candidates are competing in full. John McCain needs to show that he can win registered Republicans. So far, the Arizona senator has won two key primaries – New Hampshire and South Carolina – on the backs of independent voters. But in Florida and many of the 20-plus "Super Tuesday" contests on Feb. 5, the primaries are "closed" – registered Republicans only. If Senator McCain wins Florida, he will be the GOP front-runner, analysts say.
Mitt Romney, the former governor of Massachusetts, has now won three contests – Wyoming, Michigan, and Nevada – but only Michigan was competitive. Mr. Romney has yet to register in national polls as a top contender, but a win in Florida would catapult him into the upper ranks, analysts say. And his personal fortune frees him from the money challenge faced by McCain. The question for Romney is whether the message of economic optimism that played well in his native state, recession-hit Michigan, can win in Florida and beyond.
Mike Huckabee, the former governor of Arkansas and a favorite among Evangelicals, has not won a race since the Iowa caucuses. He trails in funding and organization, but in a state where about 25 percent of GOP voters are evangelical, he cannot be counted out completely.
Then there's Rudolph Giuliani. The former New York mayor spent most of 2007 atop national polls, only to see that lead fade as he gave up on the early contests and put all his focus on Florida, the nation's fourth-largest state and home to many transplanted New Yorkers. So far, at least part of the gamble has paid off: There is no clear front-runner among Republicans, and the field is crowded; add to the mix libertarian favorite Rep. Ron Paul and former Sen. Fred Thompson of Tennessee. So even a modest plurality of votes could win Florida. And win he must, analysts say.
"It's do or die for Rudy," says Del Ali, an independent pollster whose survey of Florida last week shows a tight race there. "I don't think a close second counts. But if he wins, it gets crazier than it is now."
Among the Democrats, where the race is effectively a three-person contest, Hillary Rodham Clinton gained momentum by winning the Nevada caucuses last Saturday. But perhaps more important, the demographics of the New York senator's win demonstrated in stark terms the challenge faced by her Senate colleague, Barack Obama of Illinois.
Entrance polls showed Senator Clinton beating Senator Obama decisively among women (51-38 percent), Latinos (64-26 percent), whites (52-34 percent), and voters age 60 and over (60-31 percent). Obama beat her soundly among African-Americans (83-14 percent), voters age 18 to 29 (59-33 percent), and independents (47-33 percent). Obama also beat her among men (by two points) and among voters age 30 to 44 (46-38 percent).
The third candidate, former Sen. John Edwards of North Carolina, won just 4 percent in Nevada, compared with Clinton's 51 percent and Obama's 45 percent. But Mr. Edwards is sticking out the race at least through South Carolina, his native state, which holds its Democratic primary Saturday. Two questions surround Edwards's long-shot candidacy: While he's still a candidate, from whom does he take votes away, Clinton or Obama? If he were to drop out, where do his supporters go?
If Edwards stays in until the August convention, and the delegate counts between Obama and Clinton are close, Edwards could conceivably play king- or queen-maker, throwing his delegates behind one or the other front-runner.
But for Obama to overcome the demographic deficits revealed in Nevada (and New Hamsphire), he needs to broaden his appeal among women, older voters, and Latinos. And, in a contest where race took center stage in the past two weeks, he needs to hold onto his black vote. That begins in South Carolina, where half the Democratic electorate is African-American.
"Looking ahead, Obama has to win in South Carolina, period," says Ted Jelen, a political scientist at the University of Nevada, Las Vegas. "If he can't win there, it's unlikely he can win anywhere else."
Yet if Obama does win South Carolina, as expected, the Clinton camp will argue that he won because of the heavily black population, and the victory could be discounted.
If the trend toward identity politics continues beyond South Carolina, the Democratic candidates may find themselves battling for blacks and Hispanics until a clear winner emerges. Hispanics are numerically stronger out West. African-Americans are more numerous across the South and represent more voters in northern cities.
"Hillary has an advantage in California, and Obama will have a better chance in the Northeast and the Southeast," says Tom Schaller, a political scientist at the University of Maryland, Baltimore County.