Former vice president Joe Biden is still leading the large field of Democratic presidential candidates but his lead over Sen. Elizabeth Warren has been sliced in half, a new poll out Wednesday found.

A national USA TODAY/Suffolk University poll found that 26% of likely Democratic primary and caucus voters said they would support Biden while 17% said they were planning to back Warren. The gap between the two frontrunners is half what it was in the same poll in late August, when Biden led Warren by a margin of 18 points.

Warren has been steadily rising in the polls in the last several months and is now jockeying for the lead of the field with Biden, trailed closely by Sen. Bernie Sanders from Vermont.

Biden on the Trail View All 16 Images

Sanders was third in the recent poll with 13% of voters saying they would back him. South Bend, Indiana Mayor Pete Buttigieg came in fourth at 10%.

But with more than 18% of Democratic voters still undecided and 57% open to changing their mind before the primary or caucuses, according to the poll.