A new poll from Monmouth University on Monday showed that Democratic presidential candidates Sen. Bernie Sanders of Vermont, Sen. Elizabeth Warren of Massachusetts and former Vice President Joe Biden are in a virtual three-way tie. Sanders and Warren are both getting 20% of the vote, with Biden receiving 19%. 

The poll was conducted among a subsample of 298 registered voters who prefer Democrats, with a margin of error of plus or minus 5.7%.

A Monmouth poll in June showed Biden at 32%, with Sanders at 14% and Warren at 15%. The results show that in just two months, Biden's popularity has taken a hit. 

"The main takeaway from this poll is that the Democratic race has become volatile. Liberal voters are starting to cast about for a candidate they can identify with," said Patrick Murray, the director of the Monmouth University Polling Institute.

"Moderate voters, who have been paying less attention, seem to be expressing doubts about Biden. But they are swinging towards one of the left-leaning candidates with high name recognition rather than towards a lesser-known candidate who might be more in line with them politically," Murray continued. 

Warren and Sanders are both seen as progressive candidates, running on platforms of providing Medicare government health insurance for all Americans, taking on big banks and corporations, and erasing the country's student loan burden.

Biden is considered more of a moderate, by expanding the Affordable Care Act and lowering college tuition costs. 

Biden has strong name recognition after sharing the 2008 and 2012 ticket with former President Barack Obama, who remains popular.

However, there have questions about his many gaffes over the years that seem to have carried over into 2019.

While at a campaign stop Monday, Biden said that "I'm not going nuts," in response to criticism about the gaffes.

Many pundits still consider Biden to have the best chance of defeating President Trump in 2020. He has received strong support from blue-collar Americans in the Midwest and could have appeal amongst seniors in Florida. Former Democratic National Committee chairman and former Pennsylvania Gov. Ed Rendell recently said he believes Biden will win the nomination. 

"I think in the end, Joe Biden will prevail," Rendell said. "But you can’t be sure of that. Elizabeth Warren is coming on very strong. She’s a good campaigner. [Sanders] has a resolute following...You just don’t know how this is going to shake out."

Murray said that of the Democratic candidates, Warren seems to have the highest momentum, as her personal ratings among voters continue to rise.