Bernie SandersBernie SandersSanders keeping door open on 2020 Parliamentarian deals setback to GOP repeal bill OPINION | Hey Dems, Russia won't define 2018, so why not fix your party's problems instead? MORE leads Hillary ClintonHillary Rodham ClintonTrump backers eye GOP primary challenges for Flake, Heller Franken: Trump Jr., Manafort need to testify under oath Special counsel investigators pushing Manafort for cooperation: report MORE in a major Iowa poll for the first time since September, the latest sign the Democratic primary race is tightening between the two candidates.

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The Vermont senator wins 49 percent of likely caucusgoers in the Quinnipiac University poll released Tuesday, compared to Clinton's 44 percent. It's a significant drop for the former secretary of State from last month's Quinnipiac poll, which had her up by 11 percent.

Sanders is boosted by his 30-point lead among men, a significantly higher favorability rating, and more favorable views of his character traits and values. He's seen as a better steward of the economy and climate change, while Clinton wins on foreign policy, terrorism and healthcare.

Iowa Democrats still believe Clinton has a better shot at the White House, with 85 percent picking her compared to 68 percent who think Sanders can win.

Sanders's lead is just outside the poll's margin of error of 4.4 percent.

While Sanders was polling well in New Hampshire, Clinton had remained in much better shape in Iowa, where she regularly posted double-digit margins as recently as December. But the caucus state's polling has narrowed in January, with this week's NBC/Wall Street Journal/Marist poll showing her up by just 3 percent.

A win in both states would be a major victory for the Sanders campaign as he seeks to push back against the narrative that his support doesn't translate into electability. But even if he wins Iowa and New Hampshire, he's polling significantly worse in most other states, including South Carolina and Nevada, the next two Democratic primary contests.

Former Maryland Gov. Martin O'Malley won just 4 percent of the vote in the new Quinnipiac poll, which could have a significant impact on whether he makes this weekend's Democratic debate. He needs to average above 4.5 percent in the five most recent qualifying polls; he stands at 4.4 percent when factoring in the new poll.

—This report was updated at 12:30 p.m.