Former Vice President Joe Biden has lost further ground in a new national poll and is now in a virtual tie for first alongside Bernie Sanders and Elizabeth Warren in the race for the Democratic nomination.

Monmouth University's poll released Monday found the two senators each clinching 20%, with the Vermont independent gaining 6 percentage points and the Massachusetts Democrat jumping 5 points since the last Monmouth poll in June. Biden followed with 19% of support in Monday's poll, dropping by 13 points since the June survey which found him with a firm lead of 32%.

Monday's survey marks the first time the former vice president hasn't led the massive Democratic primary field in a major national poll this year, according to polling data from RealClearPolitics . No other Democratic candidate amassed double-digit support in Monday's poll.

"Biden's drop in support is coming disproportionately from later states that have less impact on the process. But if this trend continues it could spell trouble for him in the early states if it undermines his claim to being the most electable candidate," said Patrick Murray, director of the Monmouth University Polling Institute.

"This could benefit someone like [Sen. Kamala] Harris, who remains competitive in the early states and could use a strong showing there to propel her into the top tier. Based on the current data, though, Warren looks like the candidate with the greatest momentum right now."

Biden lost major support over the past two months because he's seen a decline in most demographics, including among white Democrats, voters of color, college-educated voters and those without a degree. Monmouth's pollster noted that Warren and Sanders almost equally benefited from Biden's drop in support among the groups.

Monmouth's polling trends with another recent national poll that showed Biden's lead dropping as Sanders and Warren, two progressive senators, surged to form a solidified top three for the Democratic nomination.

A weekly national tracking poll from The Economist/YouGov conducted during the same time as the Monmouth survey found Biden with a narrow lead of 22%, followed by Sanders with 19% and Warren with 18%. A week prior, the same tracking poll found Biden with a 1-point lead over Warren .

But another poll conducted by CNN around that same period of time found Biden with a larger lead of 29% and Sanders and Warren much further behind with 15% and 14%, respectively.