Marco Rubio delivered a speech Wednesday in South Carolina, where is battling Ted Cruz for second place behind Donald Trump. (Alex Holt/For The Washington Post)

After more than a week of fevered campaigning, Republicans in South Carolina are on the verge of answering two of the main questions hovering over the GOP presidential race: Will Donald Trump cement his place as the party’s undisputed front-runner, and will two Cuban American freshman senators emerge as his chief challengers?

The race has resembled a three-man contest more than ever during the final push before voters head to the polls here Saturday. Trump is heavily favored to win, and Sens. Ted Cruz of Texas and Marco Rubio of Florida are furiously vying for second place.

Behind them are Ohio Gov. John Kasich, former Florida governor Jeb Bush and retired neurosurgeon Ben Carson — all of whom face increasingly difficult roads ahead if they don’t find a way to get into the top tier.

Trump, Cruz and Rubio have directed most of their fire at each other this week, trading the kind of petty insults and underhanded tactics that have come to define South Carolina’s primary. With each day, the sniping has escalated.

Cruz and Rubio spent much of Thursday attempting to position themselves as the main alternative to Trump. The mogul, meanwhile, focused well beyond the presidential race to train his ire on Pope Francis, who said anyone who wants to build walls, not bridges, “is not Christian.” Trump, who has called for a wall on the U.S.-Mexico border, deemed the remark “disgraceful.”

Ahead of Saturday’s Republican primary in South Carolina, the latest polling indicates a majority of evangelical voters back Donald Trump, yet remain bitterly divided as many question his stance on social issues ranging from abortion to gay marriage. (Dalton Bennett/The Washington Post)

Trump’s focus on the pontiff reflected both his commanding front-runner status and his willingness to lurch from topic to topic at any moment. A new round of South Carolina polls showed Cruz and Rubio running neck and neck but well behind Trump.

Rubio and his aides kicked off the day by condemning a doctored photograph Cruz’s campaign posted on an anti-Rubio website designed to tie Rubio to President Obama. Rubio also attacked Cruz’s national security record as “weak.” Meanwhile, Cruz’s campaign repeated its argument that Rubio supports “amnesty” for undocumented immigrants.

[In S.C., Ted Cruz struggles to be the winner conservatives hoped for]

At a Thursday morning campaign stop in Greenville, Rubio strategist Todd Harris handed reporters copies of the photo, which depicts Rubio and Obama shaking hands.

“The picture’s fake. It’s a Photoshop of someone else shaking hands, and it appears that it isn’t even Barack Obama,” Rubio told reporters at a stop in Anderson. “So I think this is now a disturbing pattern, guys. It’s a disturbing pattern. Every day, they’re making things up.”

Cruz’s campaign did not dispute that the photo was a fake. But Cruz’s spokesman, Rick Tyler, sarcastically thanked Harris on Twitter for directing traffic to the website, which argues that Rubio’s record is similar to Obama’s. Another aide posted what appeared to be a real picture of Rubio and Obama shaking hands.

Rubio has insisted for weeks that Cruz has been dishonest in both his tactics and his claims about Rubio’s policy positions. His argument has gotten a boost in recent days from Trump, who accused Cruz of lying during Saturday’s debate.

Rep. Steve King (R-Iowa) joined Cruz at a barbecue restaurant in Easley, about 45 minutes north of Anderson. King criticized Rubio for pushing a comprehensive immigration bill in the Senate. Rubio now favors a piecemeal approach to immigration reform but still favors a path to citizenship for undocumented immigrants.

After stumbling badly into fifth place in New Hampshire after a poor debate performance, Rubio has regained some of the steam he had picked up in the Iowa caucuses, where he finished third. He landed the endorsement of South Carolina’s popular governor, Nikki Haley, on Wednesday and released a TV ad starring her on Thursday.

[Nikki Haley: ‘Let’s make Marco Rubio the next president!’]

Cruz won Iowa and finished ahead of Rubio in New Hampshire, where he invested far less than the senator from Florida. South Carolina, with its many Christian conservative voters, was seen as fertile ground for Cruz, who has put his religious faith in the foreground of his pitch to voters.

But Cruz has faced a fierce onslaught of attacks here from both Trump and Rubio. And Trump is still the biggest attraction of all, dominating the polls and attracting huge crowds wherever he goes.

Speaking at a luxury Kiawah Island golf clubhouse, Trump declared his love for South Carolina and presented the primary as an urgent moment in the campaign. “We have a big, big deal on Saturday, so we have to get out there,” he said.

Trump easily won in New Hampshire but sounded cautious about a repeat while answering questions from reporters.

“New Hampshire was so great, and frankly we hope for the best here — we just don’t know,” he said.

At a Republican Women’s Club lunch in Greenville, Cruz said he understood why someone would support Trump — though he advised against it.

“You’re ticked off,” Cruz said. “You’re ticked off at Washington. Washington’s corrupt. And Trump feels like a way to tell Washington to go jump in a lake.”

The fiery Texan added: “As voters get closer to Election Day, you start looking at the records of candidates. You start examining them more closely.”

[Debate rips open GOP wounds, and party risks tearing itself apart]

Rubio, however, is beginning to look like Cruz’s more immediate worry. He has gained ground on Cruz in the polls over the past week and is attracting enthusiastic crowds across the state.

The race will shift quickly from South Carolina to Nevada, which holds its Republican caucuses Tuesday. What happens here could affect the outcome three time zones away, where the field has spent less time.

Another factor that could influence the Cruz-Rubio brawl is Bush. A poor showing in South Carolina, where Bush campaigned this week with his brother, former president George W. Bush, could be a fatal blow for his campaign. The natural place for Jeb Bush’s deep network of donors is Rubio’s corner.

Rubio’s team, which generally has tempered expectations throughout the campaign, has been outwardly confident this week, suggesting that internal polling and other data show he is surging.

“He may even catch second place, and if we catch on fire, we have a long-distance shot at first,” predicted Sen. Tim Scott (R-S.C.), a Rubio supporter. Scott said Rubio “will certainly do better than Jeb.”

Cruz’s campaign is arguing that anything short of a victory would be an utter failure for Rubio.

“If Marco Rubio can’t win South Carolina with the endorsements of a popular governor, a very popular junior senator and one of the most popular congressmen in the entire state” — a reference to Rep. Trey Gowdy (R) — “look at the map: Where is he going to win?” asked Cruz adviser Jason Miller.

Rubio’s team believes the race could stretch on for months. Cruz also appeared to be bracing for a lengthy and multi-front competition. At least two times on Thursday, he brought up a national poll showing him ahead of Trump.

He simply referred to Rubio, who sat in third in the survey, as the “next closest candidate.”

Zezima reported from Anderson, S.C. Jenna Johnson in Kiawah Island, S.C., contributed to this report.