The 2020 Democratic presidential candidate field seems to grow by the day, increasing the chances that someone unexpected could emerge from the pack.

But, at the moment, the most conventional candidate is ahead in the early polls. That would be former Vice President Joe Biden.

Historically speaking, though, there isn't much recent precedent for someone like Biden to win the presidency, even if he does snag the party's nomination, presidential biographer Jon Meacham Told NBC's Chuck Todd on Sunday's edition of Meet the Press. Meacham, who has written books about Franklin Roosevelt, Andrew Jackson, Thomas Jefferson, and George H.W. Bush, said that when looking at Democrats who have been elected to the Oval Office in the post-World War II era, most of them have been younger, more innovative forces.

"You can't build bridges to the past," Meacham said, arguing that there is a risk Biden will be remembered as this era's Bob Dole, a successful Republican politician who served as the party's Senate leader for over a decade and challenged — before ultimately losing to — former President Bill Clinton in 1996.

WATCH: As Biden faces backlash from opponents, can history serve as a guide for his future? #MTP #IfItsSunday@jmeacham: "There is a risk that Biden becomes the Bob Dole of this period." pic.twitter.com/9dNmTDUsQr — Meet the Press (@MeetThePress) June 2, 2019

That said, Meacham also acknowledged that President Trump defied all expectations to win in 2016, so sometimes you need to throw the history book out the window. Anything can happen. Tim O'Donnell

CORRECTION: This post originally misstated Bob Dole's role during the George H.W. Bush presidency. We regret the error.