People in Iowa, New Hampshire and Michigan made clear in exit polls that the top contenders have reasons to hope, but also weak spots to patch. Here are insights from the first rounds of voting, which have left neither Democrats nor Republicans with clear front-runners.
JOHN MCCAIN: As his victory last week in New Hampshire faded into defeat in Michigan on Tuesday, exit polls for The Associated Press and the networks found that he has been unable to overcome the problem that cost him the 2000 nomination: a failure to win the GOP's crucial conservative, religious and loyal party voters.
The Arizona senator did credibly with independents and moderates in Iowa's Jan. 3 caucuses, finished a strong first with them in New Hampshire and prevailed again in Michigan. But his Michigan margins were modest, and far smaller than when he won the state's 2000 primary.
More ominously, he has yet to carry a majority of people calling themselves Republicans or conservatives. They will dominate virtually all coming GOP contests, including South Carolina on Saturday, Florida on Jan. 29 and the Feb. 5 Super Tuesday contests in nearly half the country.
McCain has also had a tough sell with conservative Christian voters - a major force in South Carolina. Though he tied his top rivals for the lead among New Hampshire's born again and evangelical Christians, he trailed in Iowa and Michigan. He has also not done well with the many GOP voters who want a candidate who shares their values.
BARACK OBAMA: The youthful Illinois senator has a problem similar to McCain's.
He's doing well with young, independent, higher income and well-educated voters. But he's had less success with poorer, less educated and older people and with registered Democrats - important groups in Democratic contests. Gary Hart, Bill Bradley and Howard Dean all ran well among elite voters and independents - and never won the party's nomination.
Due to a dispute over the timing of Michigan's Democratic primary, most major Democratic candidates were not on the ballot Tuesday and the results meant little for Obama. But in Iowa and New Hampshire, he did best with younger voters. Even in defeat in New Hampshire, six in 10 voters under age 25 supported him - and only a third of those over 65.
He carried independents in Iowa and New Hampshire, but lost among registered Democrats in New Hampshire. In his Iowa victory, he split registered Democrats evenly with his chief opponent, Hillary Rodham Clinton. Obama also has done especially well with people earning more than $100,000 yearly and college graduates, and worst with those who make under $50,000 or haven't finished college.
MITT ROMNEY: The Michigan GOP victor used an optimistic pledge to help the state's withering auto industry to attract those who name the economy as the nation's most important problem. Yet neither the former Massachusetts governor nor any of his rivals could lay claim to the issue, a top national concern as well.
In New Hampshire, those calling the economy the top problem preferred McCain, while in Iowa they liked Mike Huckabee, who won that state - indicating there is no consensus candidate on the topic.
Even as the candidates grapple for voters' trust on the economy, Romney does best with people who say it is in good shape. In Michigan, he led McCain by nearly two-to-one with that group, but tied with him among those saying it is suffering.
With the nation facing continuing housing, job and related problems - and a recent CNN-Opinion Research Corp. poll finding six in 10 think a recession has already started - the number of people worried about the economy may only grow.
HILLARY RODHAM CLINTON: South Carolina will answer a big question for the New York senator: Which candidate will blacks support? They comprise about a fifth of Democratic voters nationally but will be about half in South Carolina's contest.
Few blacks live in Iowa and New Hampshire, but they were about a quarter of the vote Tuesday in the uncontested Democratic primary in Michigan. Though six in 10 whites voted for her, seven in 10 blacks voted "uncommitted" - and nearly all said they would have voted for Obama had he been on the ballot.
Clinton also carried Michigan's female vote - a pivotal Clinton constituency that abandoned her in Iowa but returned in New Hampshire. Yet minority women voted heavily for "uncommitted," and they will be a sizable group in South Carolina.
Clinton and Obama did about equally well among the overwhelming number of Democrats worried about the economy, although she did slightly better than him with those having the toughest time.
MIKE HUCKABEE: The former Arkansas governor and Baptist minister had overwhelming support from evangelicals and conservatives in Iowa, but they edged away from him in New Hampshire and Michigan, where he campaigned less. He trailed Romney for evangelical support in Michigan - and only a fifth of conservatives backed Huckabee there.
Seventy percent of Huckabee's supporters in Michigan were evangelicals, 73 percent attend church at least weekly and, for 85 percent, it mattered that they share religious beliefs with their candidate - tops among the major hopefuls.
JOHN EDWARDS: He'd hoped for strong union support, but that has yet to materialize, and he has not found a segment of voters he dominates. One measure of how the former North Carolina senator is outshone: In New Hampshire, he trailed Clinton and Obama among voters with a favorable opinion of him.
RON PAUL: It would be hard to prove the Texas congressman's supporters would back McCain if Paul were not in the race, but he can't be helping him. Paul has drawn nearly a fifth of voters under age 30, and small but significant shares of independents and people upset with the Bush administration and the Iraq war - all groups McCain relies on.
THE OTHERS: Former New York Mayor Rudy Giuliani and one-time Tennessee Sen. Fred Thompson, who barely campaigned in the initial contests, almost vanished from those states' radar screens. Few who worried about terrorism, Giuliani's most visible issue, backed him, while few conservatives - Thompson's label - supported him.
