Political polls can be volatile and are not always a reliable predictor of a political race’s outcome. (See: The 2016 Presidential Election.) However, taken in aggregate, polls can reveal important trends and signal if a particular candidate is gaining or losing momentum.

In the case of Joe Biden, his up-and-down performance in recent polls reveals a cause for worry. While the former vice president claims that he is the most likely candidate to beat President Trump in a general election, thereby making him the “most electable” candidate among Democrats, he is not showing that he can consistently hold his ground against left-leaning Sens. Bernie Sanders and Elizabeth Warren.

A new survey, released by Monmouth University on Monday, Aug. 26, shows Biden, Sanders, and Warren in a virtual three-way tie. Both Sanders and Warren polled at 20 percent, with Biden at 19 percent. This represents a significant change from Monmouth’s last poll, released just two months ago in June, where Biden had a solid double-digit lead of 32 percent to Warren’s 15 percent and Sanders’ 14 percent.

Biden’s campaign is calling this latest Monmouth poll result an outlier, while pointing to other polls that still name him as the party’s consistent front-runner. However, even Joe Biden’s campaign cannot deny that the race for the 2020 Democratic nomination has become increasingly volatile.

Biden is running as the Democrats’ seasoned statesman. He’s the candidate that’s been in Washington for the better part of 50 years, the veteran who knows how to get things done. Biden is also the most centrist candidate—really the only centrist candidate—in a field where others openly embrace the idea of nationalizing one-third of the U.S. economy and giving government-subsidized health care to unregistered immigrants.

Biden is not the candidate who is going to wow voters with new ideas, a fresh voice, and a message about change. He’s the conventional candidate, largely unoriginal, whose greatest political achievements have come at the side of President Obama—who, incidentally, still has yet to endorse his candidacy.

This is why former Vice President Biden is vulnerable. He’s already a known commodity, while other candidates are still emerging and gaining ground.

What will happen when Sanders or Warren gets out of the other’s way?

Biden’s biggest rivals, Sanders and Warren, hold very similar political views. Sanders proudly wears the label of a “democratic socialist,” a moniker he lived up to last week when he introduced details of his Green New Deal, a radical proposal that would nationalize as much as one-third of the American economy. Yet, surveys taken within the Democratic Party identify Warren to be at least as far left as Sanders.

Both candidates support Sanders’s “Healthcare for All” initiative. Both support more liberal immigration policies, including decriminalizing undocumented migrants who pass through American borders. Both want to restructure the American economy and support a number of progressive social reforms. Taken together, their popularity vs. the other candidates shows that the energy of the Democratic Party really lies on the far left of the political spectrum.

During the primary process, one of these two candidates will inevitably need to step aside to make way for the other. When that happens, it will be interesting to see how much of the yielding candidate’s support base moves over to back the other candidate. If the majority of Bernie supporters eventually throw their support behind Warren, or vice versa, then Biden is very likely to be overtaken and lose the nomination.

And this is why the former vice president should worry.