(CNN) A surging Sen. Elizabeth Warren is challenging Joe Biden's dominance in the race for the Democratic nomination, standing at 22% to the former vice president's 20% in a new CNN/Des Moines Register/Mediacom poll of likely Iowa caucusgoers.

Warren's improved standing overall in the Iowa poll comes on the heels of a stronger showing in recent national polls and is bolstered by an increasing positive favorability rating (75% have a favorable view, the best in the field, and she is one of only four candidates who have improved their net favorability since the June CNN/DMR poll), as well as a growing percentage of likely caucusgoers who say she is either their first choice, second choice or someone they are actively considering. All told, 71% are at least considering Warren's candidacy, ahead of the next best candidate on that score by 11 points (Biden at 60%). Her supporters are also more enthusiastic than those behind Biden (32% of her backers are extremely enthusiastic vs. 22% for the former vice president).

The Massachusetts senator appears to be gaining ground primarily at the expense of Sanders. She holds the support of 32% of those who say they caucused for Sanders in 2016 (Sanders himself stands at 25% among that group), stands at 48% among those who consider themselves "very liberal," and for the first time in CNN/DMR polling on the race, has edged ahead of Sanders among those under age 35 (27% back Warren, 22% Sanders).

Biden's core backers remain behind him in largely the same way they were in the previous poll. Among seniors, he is the first choice of 35%, about the same as earlier this year. And he remains above 30% support with moderate and conservative likely caucusgoers.

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