Clinton, Sanders in statistical dead heat in Iowa
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A new poll shows Hillary ClintonHillary Rodham ClintonPeggy Noonan: ‘Weak and sniveling’ Trump is not as strong as his wife Sessions says he doesn't regret recusing himself from Russia probe Judiciary Committee Republicans want a second special counsel: report MORE and Bernie SandersBernie SandersLive Coverage: Senate votes down 'skinny' ObamaCare repeal Sanders: Senate healthcare fight 'totally bananas' Overnight Defense: Military won't lift transgender ban until Trump sends directions | House passes national security spending | Russian sanctions bill heads to Trump MORE running neck and neck among likely Democratic caucusgoers in Iowa.

Clinton has 48 percent support, compared to Sanders’s 45 percent, in the Hawkeye State, within the margin of error of the NBC News/Wall Street Journal/Marist poll.

The Vermont senator widened his lead over the former secretary of State to 19 points in New Hampshire, however, a 15-point jump from a similar poll earlier this month. New Hampshire will vote on Feb. 9.

Clinton remains dominant in South Carolina, with a 37-point lead over the Vermont senator, 64 percent to 27 percent.

Former Maryland Gov. Martin O'Malley holds the third spot in all three states, pollsters also found, with 3 percent in Iowa, 2 percent in New Hampshire and 2 percent in South Carolina.

Clinton and Sanders's tight race in Iowa — which holds its caucuses on Feb. 1 — could give the victor a decisive early advantage in the Democratic race for president.

"If Clinton carries Iowa, she can absorb a defeat to Sanders, who has a home field advantage in New Hampshire, and then move on to South Carolina," said Lee Miringoff, director of the Marist College Institute for Public Opinion. "If Sanders carries Iowa and then New Hampshire, this contest will, indeed, be a marathon."

Thursday's poll additionally found that 7 percent of likely voters are undecided in South Carolina, which will hold the fourth Democratic contest, on Feb. 27.

Mark Hensch contributed to this report.

Updated at 7:33 a.m.