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Bernie Sanders is becoming an increasingly big issue for Hillary Clinton.

Clinton may be the frontrunner in the Democratic race for the presidency, but Sanders has been putting up an unexpectedly robust challenge against her, and it appears to be paying off for him.

On Sunday, NBC News released a poll showing Sanders opening up a healthy lead over Clinton in the crucial state of New Hampshire.

From NBC:

The Vermont senator gets the support of 41 percent of Democratic voters, Clinton gets 32 percent and Vice President Joe Biden gets 16 percent. No other Democratic candidate receives more than 1 percent…without Biden in the race, Sanders' lead over Clinton in the current survey increases to 11 points, 49 percent to 38 percent.

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Sanders has been drawing the biggest crowds of any candidate, Republican or Democrat, in the 2016 race. Meanwhile, Clinton continues to be dogged by the controversy surrounding her use of a private email server. On Friday, she said that she was "sorry" for causing any confusion among voters over her email, but insisted that she had acted fully within the law.

In a sign that she expects possible trouble in both New Hampshire and in Iowa, the New York Times reported Sunday that Clinton's campaign is looking to the South to fend off Sanders:

Mrs. Clinton’s advisers, struck by the strength of Senator Bernie Sanders in those two states, have been assuring worried supporters that victories and superdelegate support in Southern states will help make her the inevitable nominee faster than many Democrats expect.