(CNN) Something has happened to Bernie Sanders over the last few months in the 2020 presidential race. Or, more accurately, not happened.

Sanders, the self-described Vermont Democratic Socialist, is stuck in neutral. And that's a very bad place to be with the day when actual voters will cast votes is getting closer and closer. (Just 138 days until the Iowa caucuses!

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But, that isn't even the real story for Sanders out of that poll! It's actually this: While Biden gained five points from a similar poll in July and Warren gained six, Sanders' support stayed right where it was -- up only a single point from the summer, which is a statistically insignificant change.

And this is FAR from the only poll that shows the Sanders' stagnation.

In the spring of 2019, Sanders was the clear 2nd choice to Biden. In fact, all the way through early June, Biden and Sanders were the only two candidates to consistently poll in double digits in the RCP average. But, since the summer, Sanders has flatlined. His average support has never gone higher than 17% and never gone lower than 14.8%.

Sanders, due to the loyalty of his most ardent supporters, has always had a very high floor in this race -- probably right around 14%. But, what's happened over these last few months is the emergence of what looks like a pretty low ceiling for Sanders' support in the 2020 race.

So, what gives?

The short answer is that there are just a whole lot more options for voters in this election than there were in the 2016 Democratic primary race.

In that race, Sanders was the only alternative to Hillary Clinton. If you didn't like Clinton -- and there were lots of Democrats in that camp -- you were voting for Sanders. Period. And, Sanders clearly benefited from that dynamic -- staying in the race all the way through the final primaries in June before bowing out amid Clinton's insurmountable delegate lead.

In 2020, there are 20 candidates still in the race . 20! Which means that if you don't like, say, Biden, who is currently the frontrunner, there are lots and lots and lots of other options not named "Bernie Sanders" to choose from.

One in particular -- Warren -- is another reason that Sanders seems to have hit a bit of a wall.

Warren's candidacy looks a lot like Sanders': Unapologetic liberalism in support of broad structural change in politics and society. Warren supports Sanders' "Medicare for All" proposal. Warren and Sanders are both prominent supporters of the Green New Deal. They both want to tax the wealthy and corporations to pay for their proposals.

The problem for Sanders is that Warren is surging on that message while he, well, isn't. In the Real Clear Politics polling average , Warren is now clear of Sanders for 2nd place -- and the NBC-WSJ poll put her at 25%, the highest she's been in any hypothetical national polling of the primary.

What explains Warren's rise on what is, for all intents and purposes, Sanders' message? Voters like new faces! Sanders ran in 2016 -- and lots of Democratic voters flocked to him. But, for many of those voters, it may be a been-there-done-that situation. Warren is, in their minds, Bernie 2.0 -- the same proposals in a more voter-friendly (and electable) package.

And that gets at the more fundamental problem for Sanders. The people who love him love him exactly because he doesn't care about how he looks (perennially unkempt hair) or what he sounds like (very, very shout-y). Those traits make him authentic and different to them. They love him because of it, not in spite of it.

But, those same Sanders-ian traits may be the thing keeping him from growing his support in the 2020 race in any meaningful way.

If you are a Democrat and are relentlessly focused on nominating someone who can beat President Donald Trump -- and all polling I've seen suggests that's the dominant motivation for a majority of party voters -- then Sanders' nutty-professor profile may keep you from jumping on board.

There's evidence that Sanders' stall in polling is having an impact. Earlier this week, he replaced his New Hampshire state director -- never a great sign with the primary set for mid-February. On Monday he lost the endorsement of the Working Families Party to Warren. (Working Families endorsed Sanders in 2016.) And on Wednesday, Sanders' Iowa state director confirmed to CNN that their political director in the Hawkeye State is no longer with the campaign.

Now. It's better to have these struggles in the fall of an off-year rather than in the middle of primary season next late winter/early spring. And, because of the loyalty of Sanders core backers, it's hard to imagine him slipping from relevance anytime soon.

But, his struggle to attract support beyond that hardcore, uh, core is real -- and responsible for the current Sanders' stagnation in the race.

UPDATE: This story has been updated to reflect the departure of Sanders' campaign political director in Iowa.