Where do Democrats stand in 2016 polls before their first debate?

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As the 2016 Democratic presidential candidates steel themselves for their first primary debate on Tuesday, former Secretary of State Hillary Clinton continues to lead the field in national polls - but Vermont Sen. Bernie Sanders is close behind, and Vice President Joe Biden, who's still deciding whether to run and will not be on the debate stage Tuesday, could alter the landscape significantly if he enters the fray.

A CBS News poll released Sunday found Clinton ahead of Sanders by 19 points, 46 to 27 percent, among Democratic voters nationwide, with Biden coming in third at 16 percent. All the other candidates were at two percent or less. Without Biden in the mix, the poll showed Clinton expanding her lead, besting Sanders 56 to 32 percent.

A Fox News poll released Tuesday found similar results, with Clinton in the lead at 45 percent, Sanders in second at 25 percent, and Biden in third at 19 percent. All the other candidates, again, were below two percent. With Biden left out, Clinton took a larger lead over Sanders, 54 to 28 percent.

In some of the early primary and caucus states, though, Clinton's lead is more questionable. In Iowa, for example, an NBC News/Wall Street Journal poll at the end of last month found Clinton ahead of Sanders, 33 to 28 percent. But in a CBS News/YouGov Battleground Tracker poll released in mid-September, Sanders led Clinton, 43 to 33 percent.

In New Hampshire, Sanders has a fairly clear lead. The NBC News/Wall Street Journal Survey found Sanders ahead of Clinton, 42 to 28 percent. A CNN/WMUR poll released in mid-September put Sanders at 46 percent in the Granite State, and Clinton at 30 percent. And in the CBS News/YouGov Battleground Tracker, Sanders was ahead, 52 to 30 percent.

In South Carolina, though, Clinton continues to dominate the field. A CNN/Opinion Research Corporation poll released last week found her ahead of Sanders, 49 to 24 percent, in the Palmetto State. The CBS News/YouGov Battleground Tracker found Clinton leading Sanders by a similarly large margin in the state, 46 to 23 percent.

Of course, each poll is only a snapshot - the race can change dramatically in an instant, and the first debate might be just such an inflection point. Will Clinton dispel the lingering doubts about her record and her candidacy? Could Sanders turn in a performance that helps convert the enthusiasm surrounding his bid into a more durable national infrastructure? Will one of the other candidates be able to capitalize on the opportunity the debate provides to emerge from bottom of the pack? And could what happens on the debate stage push Biden to get in - or stay out?

We'll have more answers to these questions soon enough. In the meantime, tune into the Face the Nation Twitter feed for live updates on tonight's debate.

(More information on each poll mentioned in this story, including sample size, survey dates, and margin of error, can be found here.)