(CNN) -- Voters will head to the polls and caucus sites Saturday in South Carolina and Nevada, contests that could propel two candidates to front-runner status in this year's wide-open presidential races.
A win in the South Carolina Republican primary could give one of the candidates a hand up in a race that, so far, has produced three different winners in three major contests.
Historically, the path to the Republican nomination has gone through South Carolina, which relishes its role of being the political gateway to the South.
"South Carolina is the state where the Republican base passes judgment on the candidates," CNN senior political analyst Bill Schneider said. "If conservatives are going to rally behind any single contender, we'll see that happen in South Carolina."
Recent polling in South Carolina has Sen. John McCain as the front-runner in the state. An American Research Group poll conducted January 15-16 had McCain leading at 33 percent, followed by Mike Huckabee, the former Arkansas governor, at 23 percent.
Mitt Romney, the former Massachusetts governor, was at 20 percent, and former Sen. Fred Thompson was at 13 percent, the poll found. All other candidates were in single digits. The poll's margin of error was plus or minus 4 percentage points.
No GOP candidate has gone on to win his party's nomination without winning South Carolina since Ronald Reagan won there in 1980, but, with the race so volatile, that may not hold true this year.
"Right now, conservatives are split. Economic conservatives like Mitt Romney, social conservatives like Mike Huckabee, and military conservatives like John McCain," Schneider said. "They could end up just as divided after the South Carolina vote."
Democrats will shift their focus west to their caucuses in Nevada. The race there will be very different from the earlier contests back East.
A Las Vegas Review-Journal poll conducted by Mason-Dixon suggests that the Nevada caucuses will come down to a race between Sen. Hillary Clinton, the winner of the New Hampshire primary, and Sen. Barack Obama, the winner of the Iowa caucuses.
The poll, conducted January 14-16, had Clinton leading Obama 41 percent to 32 percent. Former Sen. John Edwards came in third at 14 percent. All other candidates were in single digits. The poll's margin of error was plus or minus 4.5 percentage points.
"The Democratic candidates are battling for momentum going into Super Tuesday," February 5, Schneider said. "Obama had it after Iowa. Clinton had it after New Hampshire. If Obama wins Nevada, January 19, and then South Carolina a week later, he'll have the 'Big Mo.' "
"Clinton's best shot is to win Nevada," Schneider said. "That way, she can split the week with Obama, who is likely to win South Carolina on a strong African-American vote. What would that mean? Probably 'No Mo' going into Super Tuesday."
Nevada has a heavily unionized work force, and union support could be a huge factor in the outcome of the race there. Obama gained an advantage when he was endorsed by the 60,000-strong Culinary Workers Union, whose members work in Las Vegas and Reno casinos and hotels as kitchen workers, cocktail servers, housekeepers and bellhops.
Obama's cause gained a boost Thursday when a judge allowed nine at-large caucus sites in Las Vegas casinos. The state teachers union went to court to challenge the sites, arguing that on Saturday night, they will give the roughly 200,000 workers on the Las Vegas strip an unfair advantage over other voters who have to work that night.
The teachers union has not endorsed Clinton, though some of its members and leaders are backing the former first lady. The Clinton campaign said it had no involvement in the lawsuit.
Nevada will also be the first test of the Democratic candidates' support among Hispanics. Democrats moved Nevada's primary up, in part, to showcase the Hispanic vote, and all the major Democratic candidates are vying to capture it.
Nevada has the fastest-growing Hispanic population in the nation, with one out of every five residents now identifying themselves as Hispanic, according to the U.S. Census Bureau. Hispanics make up 12 percent of Nevada's eligible voters, compared with 9 percent of eligible voters nationally.
"If you have to win this state, and if this is the state the Democratic Party set up as the example of the influence of Hispanic voters in the party, you have to be able to show you can win Hispanic support," said Adam Segal of the Hispanic Voter Project at Johns Hopkins University.
Given that Nevada has never played such a prominent role before, nothing is a sure bet for the candidates, and it's not clear if the state party will be able to handle the increased number of voters who are expected to caucus. Some 9,000 turned out for the 2004 caucus, which then was much later in the election cycle.
Even with volunteers working multiple shifts on the phones and out in the field, the campaigns say they wonder if they can count on Nevadans to come out and vote.
"People aren't used to being one of the states that decides," said Adam Bozzi, a Nevada press secretary for Edwards. "We've had to explain to them what a caucus is. We've had to explain to them what they have to do to caucus and why it's important. So we don't know what turnouts are going to be."
Nevada Republicans will also hold caucuses Saturday, and Romney is campaigning hard there, while the other Republican candidates have kept their focus on South Carolina. Even though the Republican party cut in half the number of delegates the state party can send to the national convention as punishment for moving its caucuses to Saturday, Nevada has more delegates at stake than South Carolina.
In a presidential race that's increasingly coming down to who has the most delegates, a win could help Romney, who is expected to benefit from Nevada's large Mormon population. E-mail to a friend