One week before the opening Iowa caucuses, former Vice President Joe Biden leads a fluid Democratic field, a new Suffolk University/USA TODAY Network Poll of the state finds.

In the survey taken Thursday through Sunday, Biden was backed by 25% of likely Democratic caucusgoers, ahead of Vermont Sen. Bernie Sanders at 19%; former South Bend, Indiana, Mayor Pete Buttigieg at 18%; Massachusetts Sen. Elizabeth Warren at 13%; and Minnesota Sen. Amy Klobuchar at 6%.

But an additional 13% were undecided, and 45% of those with a preference said it was still possible they might change their minds. That underscores the possibility of more shifts in a state where Biden, Sanders, Warren and Buttigieg have each been ahead in one poll or another in recent months.

The Iowa outcome typically narrows the presidential field – costing also-runs the funds and standing to continue their campaigns – while providing a burst of momentum for the winner heading into the New Hampshire primary and other contests that follow.