Voters in South Carolina are casting ballots Saturday for the U.S. Republican presidential primary, a critical contest for a party eager to find a challenger to unseat President Barack Obama.

Recent polls suggest former congressman Newt Gingrich is a surging challenger to former Massachusetts governor Mitt Romney, who won last week's key primary election in New Hampshire and nearly tied in the first nominating contest in Iowa.

Despite the win, Romney, a multimillionaire investor, has struggled in recent days following questions about his personal wealth, and his acknowledgement that he pays taxes at a much lower rate than most Americans.

Gingrich, meanwhile, has been surging in polls after two well-received debate performances. He could also benefit from Texas Governor Rick Perry's decision to drop out of the race Thursday and endorse him.

Romney tried to lower expectations, saying “to be in a neck-and-neck race at this last moment is kind of exciting.”

The stakes are high in Saturday's primary, as a clear choice for a nominee typically surfaces within the first several contests.

Gingrich and Romney were expected to cross paths early Saturday during campaign events scheduled at the same time in Greenville, South Carolina.

The two other Republican contenders, former Pennsylvania senator Rick Santorum and Texas Congressman Ron Paul, are expected to battle for a third-place finish.

During a volatile debate Thursday night, the final four contenders argued over jobs, health care and leadership.

Gingrich became angry when the first question directed to him was about allegations by his ex-wife that he asked for an “open marriage” in 1999 to accommodate an extramarital affair. The former speaker of the House of Representatives has admitted to romantic affairs during his first two marriages, but he denied the version of events recounted by his second wife.

For many American voters, the U.S. economy and high unemployment are the big issues in the presidential election in early November. President Obama has no competition for the Democratic Party's nomination, but many political analysts say the sluggish economy means he faces a tough race against the eventual Republican nominee.