Republican presidential frontrunner Mitt Romney has received a major boost in his White House bid after GOP rival Jon Huntsman quit the race, endorsed Romney as candidate and called for an end to the "toxic" in-fighting that is damaging the Republican cause.

Although Huntsman attracted only about 4-5% of voters, these mainly moderate voters are expected to go to Romney, and that slim margin may make the difference in Saturday's increasingly nasty South Carolina primary.

Huntsman, a moderate who fought a limp campaign that left him at the back of the polls in South Carolina, endorsed Romney, in spite of having described him last week as unelectable against Barack Obama and lacking core conservative principles.

"I believe it is now time for our party to unite around the candidate best equipped to defeat Barack Obama. Despite our differences and the space between us on some of the issues, I believe that candidate is governor Mitt Romney," Huntsman told a press conference in Myrtle Beach.

Romney is on course to add South Carolina to his victories earlier this month in Iowa and New Hampshire, leaving him well on his way to becoming the Republican nominee to face Obama on 6 November.

With Huntsman out, the field has narrowed to five, with Romney up against four right-wingers: former speaker Newt Gingrich, former senator Rick Santorum and Texas governor Rick Perry, and the maverick libertarian Ron Paul. South Carolina represents their best hope of halting Romney.

An average of recent polls in South Carolina by the Real Clear Politics website puts Romney on 30%, Gingrich 22%, Paul 15%, Santorum 14%, Perry 6% and Huntsman 5%.

Responding to a general feeling among grassroots conservatives that the infighting will eventually help the Democrats get Obama re-elected in November, Huntsman said: "At its core, the Republican party is a party of ideas. But the current toxic form of our political discourse does not help our cause. Today I call on each campaign to cease attacking each other."

The chance of any of them heeding Huntsman's plea to end the infighting are close to nil in a campaign in which the candidates are spending millions of dollars in personally abusive negative ads. No state in recent US history has been so swamped with negative ads as South Carolina.

Showing no sign of toning down the debate, Santorum, the most socially conservativate of those remaining, on Monday described the ads as "gutter politics", called for an end to "smear campaigns and these smarmy robocalls" and accused Romney of spreading misinformation through "henchmen" on super-political action committee (super pacs), groups of wealthy supporters funding the ad campaigns.

Rightwing leaders, worried that Romney is going to squeeze through in South Carolina because of the party's internal divisions, have called on two of the three – Gingrich, Santorum or Perry – to quit and unite behind one anti-Romney candidate. But there is no sign of that happening before Saturday.

Eric Metaxas, the conservative author who is supporting Santorum, expressed hope that God would intervene to persuade Gingrich and Perry to quit before Saturday.

"I do not believe Mitt Romney is electable. He does not connect with those living in the Rust Belt or with Reagan Democrats," Metaxas said.

Normally, Romney would have been at the press conference side-by-side with Huntsman. But in spite of sharing a similar background – they are both Mormons – the pair do not get on, and it might have been embarrassing for Romney facing questions from reporters about what Huntsman has said about him in recent weeks.

Huntsman has promised to make robo-calls backing Romney for use in South Carolina.

Romney, in a suggestion of coolness between the two candidates, saluted him, but in a written statement that was extremely brief. "Jon ran a spirited campaign based on unity not division, and love of country. I appreciate his friendship and support," he said.

Huntsman appeared at the press conference surrounded by his wife and daughters, the Huntsman girls, who became an internet sensation for a series of spoof ads. His father, who has a personal fortune of about $900m, sat on the sidelines. Although Huntsman has left this race, he is expected to try again in 2016.

The Democrats are enjoying the spectacle of the Republicans tearing themselves apart and providing lots of lines for use in the general election.

David Axelrod, Obama's chief strategist, in a tweet, wrote that Huntsman's switch of support to Romney noted that Huntsman had once called Romney "a perfectly lubricated weather vane," who has "been on three sides of every major issue."

At a prayer meeting attended by candidates in Myrtle Beach, Karen Walto, 62, mirrored the views of many Republicans still swithering over who to support. She started the meeting firmly behind Romney, regarding him as the most electable in a general election, but, impressed by Santorum, who made a case for traditional familiy values, she was reviewing her position.

"I have not changed my mind but I am reconsidering. I am leaning towards Santorum. He is nothing like what you see on televison or in debates," Walto, a nurse from Tega Cay, South Carolina, said.