From The Des Moines Register: Clinton grows lead to 9 percent (December 14, 2015 video) [View all]
Sometimes if one has questions, it is good to go right to the home source:
Note the use of the word "grows" in the paper's headlines.
Keep in mind that our own DU Iowans have posted that many Iowans do not make up their minds until the day of the caucus. Additionally, Republicans who choose to vote for Sanders can change their registration the day of the caucus.
It is interesting the commentators noted that if Warren chose to join Sanders, that would cut the lead down to 6 points.
IMHO, Sanders will attract a lot of Independent voters. Check out this link on Bernie Sanders' secret weapon:
There are more registered Independents than either Republicans or Democrats:
Note: this chart is from the link noted above.
The article itself makes some very significant arguments:
A poll of only Democratic Leaning Independents would likely lead to even greater support for Bernie Sanders. It is therefore likely that at least two out of three Democratic Leaning Independents support Bernie's positions on major issues more than enough votes to overcome Hillary's lead within the Democratic party.
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Independent Voters Want an Independent President
As a final factor, Independents have long called for an Independent President not beholden to Wall Street or to the power brokers in either political party. Bernie Sanders has not only called himself an Independent, but he is the longest serving Independent to ever be elected to Congress. Many Independent voters would support Bernie because he is like them a true Independent. This makes Independent voters Bernie's secret weapon. If Democratic Leaning Independents turn out and vote for Bernie in the 2016 Democratic Primaries and Democratic Caucuses, Bernie will be the Presidential Nominee of the Democratic Party in 2016 and he will be the next President of the United States.
Equally important, however, is the fact that four of Bernie Sanders' salient campaign issues are issues equally important to Independents: campaign finance reform, restoring a fair tax structure, reducing income inequality and regulating or breaking up too big to fail banks. (Paraphrased from the Bernie's Secret Weapon article presented by the Washington for Bernie 2016 campaign, specifically David Spring M.Ed.)
Factoring in all of the above plus the addition of some Republicans choosing to support Sanders, and one is left with the obvious hope that Bernie Sanders can indeed win this race, and sorry to be trite, but it ain't over until it's over, and don't allow anyone to convince you otherwise.