COLUMBIA, S.C. – Voters in South Carolina’s Republican presidential primary on Saturday could determine Donald Trump’s strength as a front-runner and whether a mainstream politician will emerge to challenge him in an increasingly testy race.

Out West, Nevada’s Democratic caucuses represented the first test for Hillary Clinton and Bernie Sanders in a more racially diverse state than the opening contests in Iowa and New Hampshire, which the candidates split. Her team nervously anticipated a tight race after long viewing Nevada as the place to start pulling away from Sanders.

For both parties, the 2016 election has laid bare voters’ frustration with Washington and the influence of big money in the political system. The public mood has upended the usual political order, leaving more traditional candidates scrambling to find their footing.

No candidate has shaken the establishment more than Trump. The billionaire businessman spent the week threatening one rival with a lawsuit, accusing former President George W. Bush of lying, and even tangling with Pope Francis on immigration.

Yet South Carolina was seen as his state to lose.

That prospect alarms rival Jeb Bush, the former Florida governor trying mightily for a strong showing in the first Southern state to vote.

“Trump can’t win, plain and simple,” Bush told reporters outside a polling place in Greenville. “This isn’t about appealing to people’s deep anxiety, which is legitimate. He can’t be president. A ton of people would be very uncomfortable with his decisive language and with his inexperience in so many ways. … We’re living in dangerous times. I think we need someone who can be president from Day One.”

A Trump victory could foreshadow a solid performance in the collection of Southern states that vote on March 1. Victories in those Super Tuesday contests could put the billionaire in a commanding position in the delegate count, which determines the nomination.

Texas Sen. Ted Cruz banked on a well-regarded get-out-the-vote operation and 10,000 volunteers to help overtake Trump on Saturday, as well as in the Southern states that follow.

A failure to top Trump in South Carolina could puncture that strategy, though Cruz, who sidetracked briefly to Washington to attend the late Supreme Court Justice Antonin Scalia’s funeral Mass, will still have more than enough money to run a long campaign.

Marco Rubio and Bush were fighting to establish themselves as credible alternatives to Trump and Cruz, candidates some GOP leaders believe are unelectable in November.

Neither Bush nor Rubio expected to win South Carolina. But they wanted to finish ahead of one another; otherwise, there would be tough questions about long-term viability.

Rubio scored the endorsements of several prominent South Carolina politicians, including Gov. Nikki Haley, and seemed to have rebounded after a dismal debate performance two weeks ago.

Teacher Jason Sims of Mount Pleasant went for Rubio in a last-minute decision and said Haley’s announcement was “a big deal.”

“I was kind of riding the fence,” Sims said. “I trust her.”

Bush hoped his deep family ties to South Carolina – his brother and father each won two primaries here – would be a lifeline for his struggling campaign.

Bush said he was going to “work hard for the day” and that “it’s interesting that a lot of people claim they’re undecided this late.”

Also in the mix was Ohio Gov. John Kasich, who has low expectations in South Carolina. He was looking toward more moderate states that vote later in March. Retired neurosurgeon Ben Carson had a small but loyal cadre of followers.

For Democrats, the contest between Clinton and Sanders has grown closer than almost anyone expected.

Sanders, an avowed democratic socialist, has energized voters, particularly young people, with his impassioned calls for breaking up Wall Street banks and providing free tuition at public colleges and universities.

Clinton hoped to offset Sanders’ youth support by winning big majorities among blacks and Hispanics. She eyed Nevada, where one-fourth of the population is Hispanic, as the first in a series of contests that would highlight that strength.

But Clinton’s campaign has played down expectations in Nevada in recent days.

A victory for Sanders – or even a narrow loss to Clinton– would give his campaign a boost heading into the Democratic contests on Super Tuesday.

“If there’s a large turnout I think we’re going to do just fine,” Sanders told reporters in Las Vegas. “If it’s a low turnout. that may be another story.”

Democrats were to gather at 200 caucus sites, including six at Las Vegas Strip casinos so housekeepers, blackjack dealers and others with weekend schedules could attend.

Democrats and Republicans will swap locations in the coming days. The GOP holds its caucus in Nevada on Tuesday, while Democrats face off in South Carolina on Feb. 27.

Associated Press writers Jill Colvin in Myrtle Beach, South Carolina, and Ken Thomas in Elko, Nevada, contributed to this report.

Follow Julie Pace at http://twitter.com/jpaceDC and Bill Barrow at http://twitter.com/BillBarrowAP