"The drama in the Democratic race is created by a simple fact: If Hillary Clinton is going to be stopped, it's most likely to happen in Iowa," Schneider said. "If Clinton wins Iowa, she could be unstoppable. If she loses Iowa, she may also lose New Hampshire and South Carolina. Then we've got a real race on our hands."

An American Research Group poll conducted December 20-23 had Clinton leading the field in Iowa with the support of 34 percent of likely Democratic caucus-goers, while Obama received the support of 20 percent while Edwards had 19 percent. The poll's margin of error was plus or minus 4 percentage points.

However, in what may be a sign of the race's volatility, a CNN/Opinion Research Corporation poll conducted December 14-18 showed a much tighter race, with Clinton leading with 30 percent, Obama at 28 percent and Edwards at 26 percent. The poll's margin of error was plus or minus 5 percentage points.