The poll, taken Wednesday through Friday, put Biden at 18%, Warren at 17% and Buttigieg at 13% among 500 likely Democratic caucusgoers.

Those standings reflect significant changes since the Suffolk/USA TODAY poll taken in Iowa at the end of June, when Biden led Warren by double digits and Buttigieg trailed at a distant 6%. California Sen. Kamala Harris, who was then in second place after a strong showing in the first Democratic debate, has plummeted 13 percentage points and is now in a three-way tie for sixth. Vermont Sen. Bernie Sanders earned 9% support, the same number as in the June poll.

“Iowa is unquestionably up for grabs,” said David Paleologos, director of the Suffolk Political Research Center. Buttigieg “has found a lane and is accelerating toward the front of the pack, surpassing Bernie Sanders. All of this is happening while the number of undecided voters continues to grow as Democratic caucusgoers pause to reevaluate the changing field.”