Republicans are focused on South Carolina, four days before its "first-in-the-south" primary.

FOX's Rachel Sutherland reports:

Former President George W. Bush stumped for his brother Jeb in the Palmetto state:

(Bush) "We need someone who can take a positive message across the entire country." 

Meanwhile, Donald Trump is threatening to sue Ted Cruz, who was born in Canada to an American mother. If he doesn't apologize for comments he's made about him:

(Trump) "Based on what I've learned over the past few days from very top lawyers, he doesn't even have the right to serve as President."

Trump is also threatening a third party run if the Republican National Committee doesn't reel Cruz in. Cruz says he's standing by his statements on Trump's positions on abortion and gun rights.

Rachel Sutherland, FOX News.