Two average debate performances and several rhetorical blunders later, Joe Biden is still on top. But this race is just getting started, and the former vice president's lead might not be as untouchable as it seems.

A new Suffolk University/Boston Globe poll has Biden leading the pack, 5 points ahead of Bernie Sanders, 7 points ahead of Elizabeth Warren, and 13 points ahead of Kamala Harris. At 21%, Biden’s familiarity has given him an advantage few candidates enjoy.

But Biden’s front-runner status might not last. A large number of Democratic voters remain undecided. According to the poll, 20.8% of voters said they haven’t made up their mind and 58% said their vote could change in the next six months before the primary. This is bad news for Biden: There are just as many undecided voters as there are Biden supporters, which means the former vice president has his work cut out for him if he hopes to maintain his wide margin over the next six months.

Biden must prove to primary voters that his moderate platform can beat President Trump’s in 2020. To do so, he must ward off attacks from the progressive, left-leaning candidates who offer the more radical policies the party’s base wants.

Biden’s support is steady. Before and after each primary debate, he's polled at at least 30%. But Tuesday's poll is evidence that it’s still possible to shake things up in this race. And that’s exactly what Democratic voters want. The base wants a fighter and an innovator. If Biden wants to win, he'll have to prove he’s all that and more.

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