Six months ago, political junkies were deriding this year's Republican presidential race as one of the slowest and least exciting nomination battles in recent memory. Mitt Romney looked hard to beat. Newt Gingrich's campaign appeared all but dead. And several key Republicans whom many party insiders had begged to run—including Chris Christie, Mitch Daniels, Mike Huckabee and Haley Barbour— opted out of the campaign.

Yet that boring Republican race has turned into one of the most unpredictable and chaotic presidential nominating contests in party history. With voting set to begin in a little over three weeks, the race now enters its most crucial phase. Republican voters in the early states will finally have to decide who they want to represent their party against President Obama.

The next 10 days will be the most critical of the campaign, as the candidates begin to make closing arguments to Iowans before most voters tune out the campaign during the Christmas holiday...