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Bernie Sanders led with 28% support from Democrats in a statewide Iowa poll released on the eve of Monday’s caucuses.

In the Emerson College poll of likely Democratic caucus-goers, Joe Biden came in second with 21%, followed by Pete Buttigieg with 15% and Elizabeth Warren with 14%. Amy Klobuchar was fifth, with 11%.

Several factors complicate the poll results. Caucus-goers whose favored candidate doesn’t attain at least 15% support in their precinct will get to pick a fallback choice in a second round, which has the potential to remix the rankings. And this year, four competitive candidates are packed fairly closely at the top of a historically large field.

Emerson research assistant Brendan Kane said that those second choices could prove important for Sanders in particular. Almost half of Warren supporters name Sanders a second choice, Kane said, and she is “right on the edge” of the 15% viability threshold in several parts of the state.

Adding to uncertainty, two-thirds of respondents in the Emerson poll said they will definitely vote for their candidate, but one third said there was still a chance they could change their mind. Twenty-nine percent of those polled said they made up their mind in the last week or more recently.

One issue that concerns many Democrats is how the nomination process will affect the general election.

Among supporters of Warren, Klobuchar, Buttigieg and Biden, 84% to 90% said they would support the Democratic nominee even if it their preferred candidate didn’t win the primary. But only 62% of Sanders supporters said the same.

The poll of 853 likely Democratic caucus-goers was taken Jan. 30 to Feb. 2. It has a margin of error of plus or minus 3.3 percentage points.

( Updates with additional data beginning in fifth paragraph )