Voting will be over in a few hours in the Republican presidential primary in South Carolina.

The race turned tense in the past two days, as new opinion polls suggest former congressman Newt Gingrich could defeat former Massachusetts governor Mitt Romney. Just a week ago it appeared Romney, who won last week's key primary election in New Hampshire, would easily triumph in South Carolina.

But 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 has surged in the 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.

The candidates spent Saturday campaigning hard for votes, with Gingrich appealing to conservatives to support him.

“Please make sure that everybody you know goes to vote, and if they have any doubts, just remind them, I am the only conservative who has a chance to stop a Massachusetts moderate, and I need the vote of every conservative in South Carolina today.”

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

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

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.