After the 2016 presidential election, Senator Bernie Sanders was considered an obvious choice for the Democrats to have face off against President Trump in 2020. But now, with a deep race of 24 candidates, Sanders' percentage has slipped below 20 percent in the latest polls. Salon's Amanda Marcotte asked professor of political science from the University of Wisconsin-Madison, Kenneth Mayer, on "Salon Talks" the reasoning behind this shift.

"He doesn't have as clear a base as he did in 2016, when he could present himself as the single alternative to Hillary Clinton," Mayer explained. "Now, if you are a progressive Democratic voter, you have other options. You have Elizabeth Warren."

And Warren gaining ground continues to weaken Sanders' poll numbers as the only progressive. Warren's popularity rose with her plan to cancel student debt, create a wealth tax, break up big tech companies and more.

Mayer does make the point, "if it was a race between Bernie Sanders and Joe Biden, it would look more like a reprise of 2016. But now you've got Harris, you've got Klobuchar, you have a bunch of other candidates who are competing for the mantle of the progressive alternative."

Watch the video above to find out who is Sanders' biggest competition. Check out Salon's full episode with Mayer for more on how the 2020 Democratic primary debates could shape the race ahead.