Republican favourite Mitt Romney is alone at the top of the field of candidates in this week's primary balloting in South Carolina, where three of his more conservative challengers are splitting evangelical and social conservative voters.
Former Utah governor Jon Huntsman, by far the most moderate Republican vying for the nomination to face President Barack Obama in November, has dropped out of the race and given his support to Mr Romney.
That leaves just five men heading into South Carolina's critical Saturday vote, and a Romney victory would probably make his nomination inevitable. Romney previously was first in Iowa caucus voting, then in the New Hampshire primary polling last week.
While a majority of Republicans do not trust Mr Romney because of his moderate past, they have been unable to settle their support on the more-conservative alternatives: Newt Gingrich, the former speaker of the House of Representatives; Rick Santorum, a former Pennsylvania senator, or Texas governor Rick Perry. Small-government libertarian candidate Ron Paul, a Texas congressman, likewise has failed to rally sufficient support to make him a serious challenger to Mr Romney in the first primary vote in America's deep South.
Republicans now seem to be slowly converging on Mr Romney as the best of the Republican field to deny Mr Obama a second term in the White House. His incumbency is heavily weighed down by the struggling US economy.
The candidates faced a packed week of campaign events and nationally televised debates. In the past three decades, no Republican has won the party's presidential nomination without carrying South Carolina. Polls show Mr Romney, who struggled to a fourth-place finish in South Carolina during his 2008 White House run, with a comfortable lead.
The state has a large population of evangelicals and other conservative Christians, and concerns arose four years ago about his Mormon faith. But Mr Gingrich, Mr Santorum and Mr Perry all said Mr Romney, after his victories in Iowa and New Hampshire, continued to benefit from the fractured Republican field and the failure of social conservatives to coalesce around a single alternative.
That has left Mr Romney in control of the race, despite a lack of support from conservatives who are put off by his shifting stances on social issues like abortion.
Mr Gingrich said he would "reassess" his candidacy if he lost in South Carolina and acknowledged that a Romney victory would mean "an enormous advantage going forward."
The field that remains after Saturday's vote will next compete in Florida on January 31. The party does not officially name its candidate until the Republican National Convention in September.