Former Vice President Joe Biden is still leading in the latest Iowa caucus poll, but two of his fellow Democrats competing for the presidential nomination are making gains — Sen. Elizabeth Warren of Massachusetts and Mayor Pete Buttigieg of South Bend, Indiana.

While Biden is at the head of the pack with 24 percent of respondents selecting him as their "first choice," he is followed by Sen. Bernie Sanders of Vermont with 16 percent, then Warren with 15 percent and Buttigieg with 14 percent, according to a survey by The Des Moines Register and CNN. The next candidate is Sen. Kamala Harris of California, who only has 7 percent of the support.

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This is a sign of considerable progress for both Warren and Buttigieg. In a poll taken by the Des Moines Register in March (this data point comes from The New York Times), Biden had 27 percent and Sanders had 25 percent, with Warren only polling at 9 percent and Buttigieg only polling at 1 percent. Buttigieg claimed that the poll has vindicated his strategy for campaigning, saying outside of an event for Democrats in northeastern Iowa that "it shows that campaigning works. We’ve invested a lot of time and a lot of effort, not just nationally but getting to be known in Iowa, and obviously that’s led to some growth."

There was a positive data point in the poll specifically for Warren: An equal number of the voters who plan on caucusing in person, 61 percent, say that she is on their list of their options — that is, she is either their first choice, their second choice or is being actively considered – as they do for Biden. The only other candidates who are also regarded as serious options in this way by more than 50 percent of that subset of Iowa voters are Sanders (with 56 percent) and Buttigieg and Harris (with 52 percent each).

During an appearance on CNN on Sunday, Sanders told anchor Dana Bash that "what I think is that four years ago, you know, there were only two of us in the race, and we split the vote about 50% each. This time we've got a whole lot of candidates and I don't think anybody is going to reach 50%."

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He added, "We're not going to get 50% of the vote in Iowa. I don't think anybody will."

He also claimed that "I think we have a very strong chance of being the candidate who will defeat the worst president in the modern history of this country, Donald Trump."

Biden's candidacy has been harshly criticized for the former vice president's moderate positions, which some have argued put him out of touch with the party's liberal base, as well as because of criticisms that he has engaged in inappropriate touching around women (he has not been accused of sexual misconduct).

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More moderate Democrats have expressed concern about whether Sanders' left-wing views would make him unelectable against Trump.

"The election is going to be determined, most likely, in the upper Midwest, in the same battleground states that Donald Trump won last time and shocked Hillary Clinton," Marshall told Salon. "That means Pennsylvania, Wisconsin, Ohio, Michigan. You could also throw in Iowa. So does Bernie Sanders expand the map for Democrats in these places? Does he appeal to mainstream voters? And I don't see any evidence of it. I think that's the great challenge for Bernie Sanders and I'm not sure how he surmounts it."