GOP candidates hoping to win hearts and minds in South Carolina Rick Santorum, Mitt Romney, Newt Gingrich and Ron Paul planned events Friday across South Carolina. Romney gets the Virginia governor's endorsement The South Carolina primary will be held on Saturday The candidates sparred in a debate Thursday night In past decades, the South Carolina winner has gained the national GOP nod (CNN) -- The four Republican candidates for the presidential nomination cranked up their campaigns on Friday, a day before South Carolina's crucial primary. Former Massachusetts Gov. Mitt Romney, former Pennsylvania Sen. Rick Santorum, former House Speaker Newt Gingrich, and Texas Rep. Ron Paul planned events Friday across the hotbed of Southern conservatism. It has been an arduous campaign in what is an important state. Since 1980, the winners of the South Carolina GOP primary have won the Republican presidential nominations. Unlike Iowa and New Hampshire, South Carolina is staunchly red. Saturday's results will reflect conservative sentiment and be a test for Romney, seen by some voters as more moderate than the others. The race has tightened as candidates criss-crossed the state. Four polls over the past two days show Romney's long-held lead in South Carolina nearly evaporating by the time the four took the debate stage, as Gingrich has surged into a virtual tie. Gingrich on Thursday grabbed a major endorsement when Texas Gov. Rick Perry suspended his campaign and announced he was backing Gingrich. But on Friday, Romney won the endorsement of a key Southern politician, Virginia Gov. Bob McDonnell. "I'm a Southern governor endorsing Mitt Romney in the first Southern state primary," McDonnell said on CNN's "Starting Point with Soledad O'Brien." The governor of South Carolina has endorsed him as well. I think hopefully that will help him some. He's the one that's been consistent. Other candidates have been up and down. Romney's campaign said McDonnell would campaign with Romney in South Carolina on what will be a busy Friday for the candidates. Romney was planning rallies Friday in Gilbert, North Charleston and Greenville. Santorum scheduled stops in Lexington, Fort Mill, Boiling Springs and Charleston for town hall meetings and will attend the Citadel Patriot dinner in Charleston. Gingrich was to visit a children's hospital in Charleston, attend a town hall in Orangeburg, and hold an "Ask Newt" teleconference. He will also be in Mount Pleasant for a Patriots Point veterans town hall and book signing. Paul was to address the Southern Republican Leadership Conference in Charleston, and hold "whistle stops" in North Charleston, Myrtle Beach, North Myrtle Beach, Warrenville, and Greenville. The flurry of public appearances follows a boisterous nationally televised debate on Thursday. Gingrich turned his contempt for the media into one of his strongest performances yet as he chastised the moderator, CNN's Chief National Correspondent John King, for starting off with a question about his marriage. The question arose from interviews with ABC News and The Washington Post in which Marianne Gingrich, one of the candidate's ex-wives, alleged Gingrich asked for an "open marriage" before their divorce. "To take an ex-wife and make it, two days before the primary, a significant question in a presidential campaign is as close to despicable as anything I can imagine," Gingrich said. Marianne Gingrich told The Washington Post that a day after her husband told her about his affair with Callista Bisek in May 1999, the former House speaker delivered a speech titled "The Demise of American Culture" to a group of Republican women in Pennsylvania. Marianne Gingrich said in the interview. He married Bisek in 2000. Romney took the steady, cautious route that he has taken throughout the campaign, responding to his rivals only when attacked. Santorum presented himself as a more conservative alternative to Romney than Gingrich and as the candidate who could draw an effective contrast in the fall with President Barack Obama. As the debate was going on, Gingrich's campaign released his tax returns, as he had promised to do earlier. Romney was asked if he would follow his father's lead in releasing 12 years of tax returns. Romney's father, George, ran for the 1968 Republican presidential nomination. "Maybe," Romney said, before highlighting his business experience and his personal success. I didn't inherit money from my parents. What I have, I earned. As he had earlier this week, Santorum said he filed his own returns and they were at home on his computer and he would release them once he returned home. Paul said he had no intention of releasing his returns, arguing that congressional financial statements are revealing enough. The issue of electability was hotly debated. Earlier this week, Gingrich called for both Perry and Santorum to drop out of the race and support his candidacy because they didn't have the experience for a large undertaking like a presidential campaign. With Perry gone, Santorum was left to respond. "Grandiosity has never been a problem with Newt Gingrich," Santorum said, arguing that he's the steady choice and Republicans shouldn't pick a nominee who might surprise them. Paul touted his libertarian-leaning views. He said as president he would be unique in his ability to work with leaders on both sides of the aisle to protect civil liberties. The former obstetrician and gynecologist forced his way into the conversation on both health care and abortion, two issues he knows a lot about. "I thought maybe you were prejudiced against doctors and a doctor that practiced medicine in the military or something," Paul told King after a section where the other three candidates answered a question on whether a full repeal of President Obama's health care reform was truly possible. After a lengthy debate among the other three candidates over the legitimacy of criticizing Romney's political conversion from pro-abortion rights to anti-abortion rights, the crowd booed and chanted Paul's name when King moved on without letting Paul weigh in. "John, once again, it's a medical subject and I'm a doctor," Paul said.