SPARTANBURG, S.C. — The Republican presidential candidates hurtled across South Carolina on Friday to make their final, frantic pitch one day before the state’s primary, as polls showed the race tightening here after a volatile and often nasty week of campaigning.
The vote on Saturday, a critical test of organization and strength for much of the field, comes as the candidates are closing in on Donald J. Trump, who until now has held comfortable leads in the polls here. A new NBC/Wall Street Journal/Marist poll shows Mr. Trump just five points ahead, down from his 16-point lead in the state a month ago.
The poll had Senator Ted Cruz of Texas in second with 23 percent, and Senator Marco Rubio of Florida and Jeb Bush, the former governor of Florida, vying for third and fourth place, with 15 and 13 percent, respectively. Ben Carson, a retired pediatric neurosurgeon, and Gov. John Kasich of Ohio each had 9 percent.
As the Republican field winnows, nearly all of the remaining contenders need to deliver strong performances. A big victory by Mr. Trump would give him a jolt of momentum that could add a sheen of inevitability to his candidacy heading into the crucial March 1 contests, when 12 states vote, many of them in the South.
Mr. Cruz, meanwhile, is eager to prove his appeal is broader than that of the previous two Iowa caucus winners, Mike Huckabee and Rick Santorum, who struggled to attract voters beyond their base of evangelical support.
In recent days, the Texas senator’s rivalries with Mr. Trump and Mr. Rubio have turned particularly heated, producing a hail of negative advertising, semi-substantiated attacks, threatened litigation, accusations of doctored images and persistent charges of foul play among the three men.
“Twenty-one hours,” Mr. Cruz told a crowd in Myrtle Beach, S.C., on Friday morning. “That’s how long we’ve got until the polls open. Twenty-one hours.”
A strong performance by Mr. Rubio, who has been attracting large crowds and secured the endorsement of Gov. Nikki R. Haley of South Carolina despite stumbling badly in New Hampshire, could fully resuscitate his campaign as he seeks to become the preferred establishment alternative to Mr. Trump and Mr. Cruz.