AP Photo/Matt Rourke
Bernie Sanders.
In the Thursday CNN poll, Sanders grabbed 51% support among likely Iowa caucus-goers to Clinton's 43%.
The poll adds to the trend of a surge for Sanders in the early states of Iowa and New Hampshire over the past few weeks.
Though many party insiders have long expected that Sanders could win New Hampshire, the first-primary state, the close race in Iowa has the Clinton campaign scrambling with a little more than a week to go before the caucuses.
On Wednesday night, the campaign sent out a fundraising email asking for cash to compete with Sanders on the airwaves in Iowa and New Hampshire.
"I try not to pay too much attention to the polls, because right now they're all over the place - some have us up, others have us down," Clinton said in the fundraising email. "The only thing we can do right now is work our hearts out to put ourselves in a position to win."
Other Hawkeye State polls have continued to show Clinton at the top. A Monmouth University poll of likely Iowa caucus-goers conducted earlier this week found Clinton garnering about 48% to Sanders' 39%, for example. A Quinnipiac University poll earlier this month, on the other hand, showed Sanders with a 5-point lead.
Sanders faces a bigger hurdle in states beyond Iowa and New Hampshire with minority voters, who are virtually absent from the first two contests and play a large role in the nominating process. But some political strategists say the potential for Clinton to lose the first two states is inducing a state of déjÀ vu.
"The Clinton people are now facing the prospect where they could lose the first two states," a Democratic Party strategist unaffiliated with the presidential campaigns told Business Insider last week. "There are probably some people that are saying, 'Jesus Christ, not again.'"