It's the day when the most states vote and the most delegates are at stake. It can be determinative in who becomes the presidential nominee for either party.
The debate was a free-for-all brawl. Rubio and Cruz took turns taking hits against Trump, trying to portray him as a man with many insults but little substance
In any normal year, a Republican who wins big in New Hampshire and South Carolina would practically be seen as the presumptive nominee. So why isn't that happening this year?
Until recently, Corey Lewandowski was an obscure political operative. Now, he's engineered wins in South Carolina and New Hampshire for Trump's once-implausible presidential campaigns.
The former Florida governor, a son and brother of presidents, raised enormous amounts of money and spent more on TV ads than any other candidate, but he couldn't overcome a change-hungry electorate.
Democrats caucused in Nevada and Republicans went to the polls in South Carolina. We know the results — well, most of them by now — but here's what the day really looked like on the ground.