With voters heading to the polls Saturday in what is clearly a win-or-go-home contest for most of the remaining Republican presidential candidates, there is simply no way to escape the onslaught of political ads here, the majority of them negative.

Voters come home to find mailboxes packed with four-color fliers that slam one contender on the front and another on the back.

They pick up their phones and hear strangers on the other end, touting one competitor and smearing another.

They turn on their TVs to find back-to-back ads featuring grainy footage, black-and-white photos and deep-voiced narrators delivering mostly bad news.

The primary contest here is on pace to be the most expensive advertising campaign in history, with candidates bolstered by outside groups flush with millions of dollars and eager to test whether the negative advertising that worked in Iowa two weeks ago will work in a state known for dirty-tricks campaigning.

Even as the candidates have decried the negative ads on the debate stage and criticized the outside groups that have poured vast sums into such advertising, there is no sign of a let-up.

Rick Santorum is a counterfeit conservative, one Ron Paul ad charges. Newt Gingrich has more baggage than the airlines, according to a spot by the pro-Mitt Romney group Restore Our Future, which has spent $753,100 on television advertising here.

Romney is a moderate in the mold of President Obama and isn’t electable, charges another ad by the pro-Gingrich group Winning Our Future, which has bought $3.8 million in media coverage here, a spokesman for the group said.

“Why would we ever vote for someone who is just like Obama when we can unite around Rick Santorum and beat Obama?” a Santorum spot asks.

By Saturday, ad buys are set to easily outpace the $6.9 million that was spent in 2008, according to Kantar Media’s Campaign Media Analysis Group.

Strategists say that, unlike in past primaries, the air wars have been a race for the bottom right away.

“The thing that surprised me was that they didn’t start with sunsets and the wife-on-the-beach-type ads,” said David Woodard, a Republican strategist and Clemson University professor. “They went straight for the gut and it hasn’t gotten any better. It’s been intense, and the super PACs have made this thing a completely different ballgame.”

A barrage of negative advertising against Gingrich in Iowa worked well in slowing the former House speaker’s momentum, pushing him from the front of the pack to an eventual fourth-place finish in that state’s caucuses.

Gingrich’s supporters came into the Palmetto State with a scorch-and-burn approach of their own, tagging Romney as a corporate raider in a 30-minute documentary that was littered with inaccuracies. Gingrich asked that the film be altered or taken down, but it has gone viral on the Web.

Romney countered with an ad highlighting his success with companies such as Staples, Sports Authority and Steel Dynamics.

“Mitt Romney helped create and ran a company that invested in struggling businesses, started new ones and rebuilt old ones, creating thousands of jobs,” the spot says. “Those are the facts.”

The negative ads aren’t being run only on the airwaves. Campaigns also have jammed mailboxes and voice-mail in-boxes in an under-the-radar effort to reach voters in some markets.

“The reason why Romney is going to win this race is because he has carpet-bombed the upstate with phones and mail,” Woodard said. “Each night I get a testimonial from somebody I don’t know telling me how great Mitt Romney is. He is wearing out my voice mail.”

A CNN/Time poll taken last Friday through Tuesday showed a tightening race, with Romney edging Gingrich 33 percent to 23 percent among likely South Carolina voters. According to the survey, Santorum, a former senator from Pennsylvania, had 16 percent and Rep. Ron Paul (Tex.) had 13 percent. Texas Gov. Rick Perry, who announced Thursday that he is ending his bid, came in at 6 percent.

The ads have been must-see TV among some voters, who have parsed and discounted them.

“I listen to them to see how much is true and how much is false. There’s always a partial truth in all of them,” said Bob Massie, 65, a real estate agent in Powdersville who said he is supporting Romney. “But one of the ads against Mitt, they say he’s got 15 homes. He’s got three. So some of this is just out of the air.”

At Mutt’s Barbecue in Easely, Gingrich supporters said such ads are simply par for the course.

“You take it all with a grain of salt,” said Lorraine Turner, 58, chief executive of a group home. “But I’ll tell you this, if you’re in this business, you better get used to it.”

Randy and Pam Brantley said they try not to listen to the ads.

“They all have spin,” he said.

“And you don’t always know who they’re from, with the PACs,” she added.

But they conceded that the ads do provide information that can be worthwhile. For instance, they paid close attention to the ones that discussed Romney’s shifting position on abortion.

“It makes you wonder what changed. Does he really have conviction on this?” Pam said, echoing the main argument of Gingrich’s campaign against Romney.

Thursday night’s debate in Charleston could provide more fodder for ads: Gingrich turned some of his comments from a debate Monday into an ad that argues that he is the only true conservative in the race.

Chip Felkel, a Republican strategist, likened the 10 days of ad wars to what the state usually sees over the course of a six- or seven-month U.S. Senate race.

“We are getting a little war-weary,” he said. “We are at the edge, and I don’t see any indication that anybody is going to pull back. We are all looking forward to Sunday.”

Staff writers Amy Gardner and Rosalind S. Helderman in South Carolina contributed to this report.