Though many see Joe Biden and Bernie Sanders as the most likely nominees, their spots are far from secure.
Historically, early frontrunners rarely win – check that last bunch of elections.
Also, no candidate has truly broad support yet, and most voters are still undecided. 
At this point in the 2016 election (May 2015), the frontrunners in the Republican primary were Jeb Bush and Rand Paul: 
They ended up finishing in 5th and 6th place in Iowa.
For those not paying attention, Donald Trump ended up winning the nomination and went on to win the Presidency in 2016.
 Jacob Anders in the Andrew Yang for President 2020 Basecamp