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Hillary Rodham Clinton is seeing a post-debate bounce with Democrats in New Hampshire.

A new WBUR poll found 38 percent of likely voters supporting her and 34 percent supporting Senator Bernie Sanders of Vermont. Vice President Joseph R. Biden Jr., who has not yet decided on a run, is a distant third with 9 percent. The poll’s margin of sampling error is five percentage points.

The results show Mrs. Clinton, who was at 31 percent in WBUR’s previous poll, solidifying her position in the race, and they raise questions of whether Mr. Biden missed the right moment to make his move.

Mrs. Clinton has been dominating the race nationally and in most state polls, except in New Hampshire, where Mr. Sanders has been threatening an upset in the first primary election next year. While Mr. Sanders continues to outperform Mrs. Clinton in areas such as trust and empathy, she is the overwhelming favorite among Democrats looking for someone they think can win the general election.

The survey also found that Mrs. Clinton’s favorability rating swung upward from September to October. Meanwhile, Mr. Biden’s favorability declined.

“The increase in her support appears to be specific to the debate,” said Steve Koczela, president of MassINC Polling Group, which conducted the poll.

The debate was less friendly to lower-tier candidates such as Lincoln Chafee and Martin O’Malley. Their support remains near zero, and the two men saw their unfavorable scores doubled in the last month.