Clinton, Sanders in statistical dead heat in Iowa
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A new poll shows Hillary ClintonHillary Diane Rodham ClintonStopping Robert Mueller to protect us all Hillary Clinton hits Trump, pulls out Russian hat during Yale speech Giuliani: Mueller plans to wrap up Trump obstruction probe by Sept. 1 MORE and Bernie SandersBernard (Bernie) SandersHarvard law professor: Impeachment could worsen political dysfunction, polarization Gun control debate shifts to hardening schools after Texas shooting Bernie Sanders: NRA to blame for lack of action on gun control MORE running neck and neck among likely Democratic caucusgoers in Iowa.

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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.