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Senator Bernie Sanders and Donald J. Trump have opened up solid leads in Iowa less than two weeks before the state’s caucuses kick off the 2016 presidential nominating contests, according to a poll released Thursday.

The survey from CNN/ORC shows Mr. Sanders, who was trailing Mrs. Clinton significantly in early December, erasing her lead and overtaking her. Likely Democratic caucusgoers now back the Vermont senator over Mrs. Clinton by eight percentage points, 51 percent to 43 percent. Last month she was ahead of him by 18 percentage points.

The results could spell serious trouble for Mrs. Clinton, who is also trailing Mr. Sanders in New Hampshire polls. While Mrs. Clinton is leading most national polls, the prospect of losing the first two nominating contests could deal a major blow to her candidacy.

Iowa Democrats are increasingly feeling set in their choice of candidates, suggesting that there is little time left for Mrs. Clinton to regain lost ground. Mr. Sanders is currently seen as stronger on the economy and health care, while Mrs. Clinton is viewed as stronger on foreign policy.

Realizing she is under pressure, Mrs. Clinton has been criticizing Mr. Sanders for being a self-proclaimed socialist and arguing that many of his progressive policies are unrealistic.

On the Republican side, likely caucusgoers are backing Mr. Trump over Senator Ted Cruz, 37 percent to 26 percent. Senator Marco Rubio sits in third place with 14 percent, and Ben Carson, whose fortunes in Iowa have plummeted, is at 6 percent.

Several Iowa polls showed Mr. Cruz leading or tied with Mr. Trump late last year, but Mr. Trump has been performing better after attacking Mr. Cruz more aggressively in recent weeks. While Mr. Cruz is seen as stronger on social issues, Mr. Trump holds strong advantages when it comes to handling the economy, immigration, leadership and electability in a general election.

Questions have been raised about the strength of Mr. Trump’s ground game in Iowa, as his campaign looks to voters who are not traditionally active in politics to participate in the caucus process.

The CNN/ORC poll had a margin of error of plus or minus six percentage points in its surveys of Republicans and Democrats.