It seems like we say this a couple of times a week lately, but this weekend's Republican primary in South Carolina and both parties' caucuses in Nevada could mark the end of the line for some of the presidential candidates and the start of true front-runner status for others.

With wins by Senator Barack Obama in Iowa and Senator Hillary Clinton in New Hampshire, the race on the Democratic side is still wide open, as is the Republican scramble, which has seen three wins by three different candidates in three major contests so far. Here's what's at stake for all of the remaining major candidates this weekend.

Democrats

Hillary Clinton
Best case: She could roll to her third win in a row (if you count Tuesday's win in Michigan, which the Democratic National Committee doesn't) in the first Western state in the campaign and the first with a large Hispanic population.

Worst case: As of December, Clinton had a double-digit lead over Obama in Nevada, but it has since evaporated into a dead heat. A loss could slow the progress of the comeback kid.

Barack Obama
Best case: After gaining the endorsement of Nevada's largest union of culinary workers, representing 60,000 voters, he could cruise to his second major victory, setting up a potential knock-out blow in South Carolina on January 26 and in other states on Super Tuesday (February 5).

Worst case: If he doesn't take Nevada after fighting to let nine casinos and hotels get designated as caucus sites to help union members vote, the major endorsement could amount to little and he could be facing a re-energized Clinton on a serious roll.

John Edwards
Best case: Edwards is within striking distance of Clinton in recent polls, so another second-place finish could keep his campaign alive for another day.

Worst case: Without a win to date, and with most pundits depicting the Democratic race as an Obama-Clinton showdown, he could face serious doubts, and a cash crunch, if he places third.

Dennis Kucinich and Mike Gravel Best case: It's hard to say. Neither man has made any significant strides to date, but at this point, continuing to hang in there is a feat in itself.

Worst case: They could poll in the low single digits again and continue to be cannon fodder for late-night talk-show hosts.

Republicans

Mitt Romney
Best case: He has essentially punted South Carolina, after pulling his ads last week, and has instead gone all-in in Nevada, where he hopes his tough talk on the economy will resonate with Silver State voters, who include a significant Mormon population. A win would keep the momentum from Michigan going his way, while a decent result in South Carolina could further prove his electability to skeptical Evangelicals.

Worst case: If his gamble in the desert doesn't pay off and he finishes a distant third in South Carolina, that could prove he might not be able to compete in the South, and he'll be back on his heels again heading into Super Tuesday.

Mike Huckabee
Best case: He's back in his element in South Carolina, where Evangelicals make up more than half of the GOP electorate.

Worst case: The former Arkansas governor's appeal to gun-rights advocates, home-schoolers, IRS critics, Evangelicals and Confederate flag devotees in his home region falls flat and begins to fade. At press time, he was slightly behind John McCain in the polls in the first contest in the South, in a state whose primary winner has gone on to win the Republican presidential nomination every time since 1980.

John McCain
Best case: The tenacious Vietnam War veteran could triumph over the traditionally dirty tricks in the South Carolina race, avenging his 2000 defeat there and winning enough of the independent and Democratic vote in this open primary to sustain his New Hampshire steam.

Worst case: The Arizona senator could flop again in S.C. and fail to win in a neighboring Western state, providing further proof that he's struggling to win over the GOP's conservative, religious base and loyal party members, who make up a majority of voters in the upcoming contests.

Rudy Giuliani
Best case: He could at least manage to finish above long-shot Texas Congressman Ron Paul, who beat him in Michigan, and help set up his necessary win in Florida on January 29.

Worst case: Another single-digit shellacking could make him an also-ran and prove that his Florida-or-bust strategy was a big mistake.

Fred Thompson
Best case: The Alabama-born actor and former senator from Tennessee has called S.C. his last stand, and it could be his chance to finally climb out of the cellar.

Worst case: Another poor showing by the low-energy "Law & Order" star could close the case on his candidacy.

Ron Paul
Best case: The Lone Star congressman could stay competitive enough to keep the money pouring in from his rabid online supporters and continue to pull enough young, independent voters to keep McCain from gaining a head of steam.

Worst case: Another pair of single-digit showings could finally convince his e-backers that it's time to ground the airship Paul and admit that the anti-Iraq War firebrand doesn't have a chance.

Duncan Hunter
Best case: There isn't really much good news to report for the California congressman. With low or barely perceptible showings in all the races to date, the best he can hope for is that he doesn't get a goose egg.

Worst case: As in Michigan, he could come in eighth again in a seven-man field, behind "uncommitted."