Tune in Thursday at 8 p.m. ET for the CNN/Southern Republican Presidential Debate hosted by John King and follow it on Twitter at #CNNDebate. For real-time coverage of the South Carolina primary, go to CNNPolitics.com or to the CNN apps or CNN mobile website.
(CNN) -- Just before the final debate Thursday and days before the voting, Republican presidential candidates have launched increasingly strident attacks against each other and President Barack Obama in the run-up to the South Carolina primary.
Front-runners Mitt Romney and Newt Gingrich traded accusations involving honesty and tactics, while Rick Santorum criticized both men for what he called placing political expediency before ideological principles.
Gingrich and Texas Gov. Rick Perry also criticized Romney's past shift on the abortion issue, seeking to raise questions among South Carolina's powerful evangelical voting bloc.
The increasingly aggressive tone came amid signs of a tightening race in the nation's first Southern primary.
A new CNN/Time/ORC International Poll indicates 33% of likely South Carolina GOP primary voters are now backing Romney, compared with 23% for Gingrich. Romney's 10-point edge over the former House speaker is down from a 19-point lead two weeks ago.
According to the survey, Santorum is backed by 16% of likely voters, with 13% for Texas Rep. Ron Paul and 6% supporting Perry.
The five remaining candidates will take part in a debate on Thursday broadcast on CNN, with South Carolina voters going to the polls on Saturday. The state has voted for the eventual winner of every Republican presidential nomination since 1980.
Gingrich, Santorum, Paul and Perry are all hoping a Romney stumble in South Carolina will prevent the former Massachusetts governor, winner of the first two nominating contests in Iowa and New Hampshire, from effectively ending the GOP contest.
At a Personhood USA forum in Greenville, South Carolina, Gingrich said Romney took pro-abortion steps as Massachusetts governor, even though he had shifted from a pro-choice to an anti-abortion stance.
Perry, meanwhile, questioned the sincerity of Romney's embracing an anti-abortion position so late in his life.
"How do you change your position on the issue of either you're pro-abortion and then you change over to pro-life in your 50s?" Perry asked, later adding: "It is clear to most of us this was a choice for convenience. This was a decision that Gov. Romney made for a political convenience, not an issue of his heart."
Earlier, Romney challenged Gingrich's claim of being behind successful job creation policies back when Gingrich was in Congress in the 1980s and '90s, telling a campaign event at Spartanburg's Wofford College that Gingrich "taking credit for creating jobs is sort of like Al Gore taking credit for creating the Internet."
In response, Gingrich went after Romney's record, noting in an interview with CNN's "The Situation Room with Wolf Blitzer" that Massachusetts was 47th in the nation in job creation when Romney led the state as governor.
"So I'm not sure Gov. Romney really understands how government can create jobs or how government can kill jobs," Gingrich said.
Meanwhile, a pair of high-profile Romney supporters held a conference call warning of negative political consequences, if Gingrich remains part of an extended nomination fight.
The "longer (Gingrich) stays in this primary, the focus is always (going to be) Newt, and when the focus is Newt, the Republican Party loses," said former Rep. Susan Molinari, R-New York. "We do not want Speaker Gingrich to help re-elect another Democrat president," she added, referencing the fact that Bill Clinton won a second term in 1996 while Gingrich led House Republicans.
Gingrich responded by calling the assertion he helped re-elect a Democrat "stupid." Noting that he spearheaded the GOP takeover of the House in 1994, Gingrich questioned Romney's political past and questioned what the former governor did to help Republicans back in the 1980s and early 1990s.
"Who did he help elect? What was he doing during those years?" Gingrich asked.
In addition, Gingrich told a campaign crowd that he expected the Romney campaign "to be unendingly dirty and dishonest for the next four days because they are desperate."
"They thought they could buy this" election, Gingrich said. Now "they are discovering that they can't buy this. I think they have internal polls that show them losing, and I think they will do anything at any level. ... People power will beat money power and I need your help to beat Romney."
In recent days, Gingrich has repeatedly characterized Romney as a Massachusetts moderate who would have a hard time distinguishing himself from -- and defeating -- Obama in the general election.
Santorum, meanwhile, blasted Gingrich for suggesting that the former Pennsylvania senator should pull out of the race. Gingrich has insisted he's the only candidate capable of uniting anti-Romney conservatives to defeat Romney in the primaries and overcome Obama in the general election.
Ripping what he called "the hubris (and) arrogance of Speaker Gingrich" in suggesting Santorum was incapable of running an effective national campaign, Santorum noted Gingrich has only won election "in one of the heavily Republican districts in Georgia" where diversity was "nonexistent."
Taking aim at Gingrich's reputation as the intellectual in the race, Santorum said: "I don't have to be the smartest person in the room to come up with some harebrained idea every 20 minutes."
As the candidates continued to trade barbs, Romney's campaign also dealt with fallout relating to the his reluctance to release his tax returns as called for by rivals.
Perry told CNN that Romney's plan to make his 2011 tax return public in April would be too late for South Carolinians voting in Saturday's primary.
"If you're asking people to trust you to be the president of the United States, most powerful country in the world, don't you think we're smart enough and trustworthy enough for us to know how you made your money and where you spent your money, where you made your charitable contributions, for instance?" Perry said.
One of Romney's top GOP supporters also urged him to release his returns as soon as possible, arguing that full and rapid disclosure is the best strategy.
"I would say if you have tax returns to put out, you know, you should put them out sooner rather than later, because it's always better in my view to have complete disclosure, especially as the front-runner," New Jersey Gov. Christie said Wednesday morning on NBC's "Today Show."
"The most relevant information is the most recent," Christie argued.
On Tuesday, Romney caved to mounting pressure from both Democrats and Republicans, agreeing to release his returns after the Internal Revenue Service's April filing deadline. Romney, a former venture capitalist believed to be worth as much as $250 million, said he would provide voters with his most recent tax information.
Romney told reporters he "probably" paid a tax rate of 15% last year, since his income is derived primarily from investments. Most Americans' income is generated from workplace wages, requiring them in many cases to pay a higher percentage of their income to the federal government.
Romney's opponents believe the fact that the multimillionaire candidate paid taxes at a lower rate than many Americans proves he's disconnected from middle class realities. As further evidence, they jumped on Romney's assertion Tuesday that the nearly $375,000 he earned in speaking fees over the course of one year recently was "not very much."
Gingrich told reporters Wednesday that he pays a 31% tax rate.
"My goal isn't to raise Romney's taxes. It's for everyone to pay Mitt Romney's rate," Gingrich said. "Shouldn't we all have the option of a flat tax at the rate he's paying?"
Obama's team has also jumped into the fray. White House press secretary Jay Carney told reporters Tuesday that the president believes wealthy Americans "should not pay a lower effective tax rate than middle-class Americans."
When a reporter noted Romney was following the law, Carney responded, "The president believes we ought to change the law for that reason." He also noted that the tradition of presidential candidates from both parties revealing their tax returns dates back to Romney's father, George, who released 12 years of tax records when he sought the 1968 GOP nomination.
For those voters who don't meet the candidates face-to-face, the South Carolina airwaves continue to be filled with ads both from both the campaigns and from a number of groups supporting them.
So far, super PACs supporting various candidates or causes have spent at least $6 million on the South Carolina primary and more than $26 million overall, according to federal campaign records.
The groups, which were created after a pair of federal court decisions in 2010, are permitted to receive unlimited campaign contributions from corporations, labor unions and individual donors.
Traditional federal political action committees and candidate campaign committees may receive only limited contributions from individuals, political parties and other political action committees. Under federal law, candidates and super PACs are not permitted to coordinate campaign activity.
Topping the list of big-spending super PACs is Restore Our Future, run by supporters of Romney, which has invested at least $7.8 million in the Republican primary contest. Almost all of that money has gone for television ads and direct mail in Iowa, South Carolina and Florida attacking Gingrich.
A group supporting Gingrich, Winning our Future, is the second-highest spending presidential super PAC, at $4.3 million since just before Christmas. Perry has been the beneficiary of almost $4 million in spending by Make Us Great Again, while Paul has the support of at least two super PACs that have spent a combined $3.3 million.
CNN's Tom Cohen, Alan Silverleib, Robert Yoon, Rachel Streitfeld, Jim Acosta and Adam Aigner-Treworgy contributed to this report