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Gov. Chris Christie of New Jersey has been talking tough since his rivals for the Republican nomination joined him to campaign in New Hampshire this week, but it does not appear to be helping his poll numbers.

In fact, they are sinking.

An NBC/WSJ/Marist poll shows Mr. Christie’s support among likely Republican voters dropped to 4 percent, from 7 percent, in the last week, leaving him in sixth place.

Donald J. Trump, who placed second in the Iowa caucuses, remains atop the Republican field at 30 percent, virtually unchanged from a week ago. Senator Marco Rubio of Florida has surged to second, his support rising by six percentage points to 17 percent. And Senator Ted Cruz of Texas, the winner in Iowa, received a small bump, rising to 15 percent.

The poll had a margin of error of plus or minus four percentage points.

The figures are especially bad news for Gov. John Kasich of Ohio, 10 percent, and former Gov. Jeb Bush of Florida, 9 percent, who are banking on strong performances in New Hampshire.

Mr. Christie has been dedicating most of his resources to New Hampshire and has been particularly critical of Mr. Rubio in recent days. He has described the Florida senator as “the boy in the bubble” and mocked him for giving overly rehearsed speeches.

Perhaps realizing that Mr. Christie’s campaign is flailing, Mr. Rubio has offered little response to his attacks.

Meanwhile, separate polls released on Friday show Mr. Trump’s support holding firm in New Hampshire. A survey from WBUR found that 28 percent of likely Republican voters would back him, with 12 percent supporting Mr. Cruz and 11 percent supporting Mr. Rubio.

A Suffolk University/Boston Globe poll indicated that Mr. Rubio was gaining momentum after his strong showing in Iowa. That survey still showed Mr. Trump leading at 29 percent, but Mr. Rubio was just 10 points behind him.