Hillary Clinton still leads the 2016 Democratic field, but her lead has shrunk noticeably in recent months.

The reasons are Bernie Sanders and, surprisingly, Vice President Joe Biden.

Among Democrats and Democratic-leaning voters, Clinton garnered 51 percent of support in a new Monmouth University poll. However, this is down from 57 percent in June and 60 percent in April.

Independent Vermont Sen. Bernie Sanders garnered 17 percent, up from 12 percent in June and 7 percent in April. Sanders was followed by former Maryland Gov. Martin O'Malley and former Virginia Sen. Jim Webb, who each have 1 percent.

If Joe Biden were to run, the poll shows he would take away support from Clinton. Though he has not announced his intentions either way, 13 percent of voters said they would back him. An additional 12 percent of Democratic voters said they would be very likely to support Biden should he enter into the 2016 fray, and another 31 percent say they would be somewhat likely.

According to the poll, 15 percent of those polled remain undecided.

However, Clinton continues to have the best favorability ratings among Democrats. Seventy-four percent to 17 percent view her favorably, though Biden is a close second at 67 percent favorable and 17 percent unfavorable.

The poll of roughly 350 registered voters who identified themselves as Democrats or Democratic-leaning was part of a larger July 9-12 survey of 1,000 adults nationwide. The sample carries a margin of error of plus or minus 5.2 percentage points.