(CNN) The Beto O'Rourke 2020 train seems to be gaining steam. Those backing other potential candidates want donors and pundits to slow their Beto roll, however.

Among the most prominent critics are supporters of Vermont independent Sen. Bernie Sanders who believes the Texan isn't progressive enough

Sanders fans have at least some reason to worry. O'Rourke or really any other candidate with outsider appeal could eat into Sanders' base of young, independent-leaning voters.

If Sanders is going to win in 2020, he'll need to hold onto his support from 2016. He won more than 40% of the national primary vote, though he lost by low double-digits to Clinton nationally.

To answer whether Sanders can keep his backers, we have to understand what drove voters to him. Was it the fact that he was the progressive alternative to the more mainstream Hillary Clinton? If that were the case, O'Rourke could be vulnerable to attacks about his voting record.

O'Rourke has voted with President Donald Trump 29% of the time, while Sanders has voted with him just 14% of the time. Put another way, O'Rourke has voted more often with Trump than 80% of House Democrats. Only three senators have voted with Trump less often than Sanders.

Sanders' appeal in 2016, however, was mostly not based on ideology. You can see this best by looking at the 27 states with an entrance or exit poll . Among the approximately 25% of the electorate who identified as "very liberal", Clinton and Sanders won about an equal share of voters. That means Sanders only did slightly better among very liberal voters than he did among all voters.

So what was driving most voters towards Sanders? Remember, Sanders was not just a liberal alternative, he was seen as outside the party establishment (i.e. independent).

Sanders won the approximately 25% of people who identified as independents (instead of Democrats) in the average caucus or primary state by about 29 points. That's nearly 30 points higher than Sanders won by among very liberal voters. He won independents in 24 of the 27 states, even though he lost overall in 20 of the 27 contests polled.

It was this ability to separate himself out from the usual Democratic brand that likely helped spur a very large age gap in the primary.

Among the approximately 30% of the Democratic primary electorate who were under 40, Sanders beat Clinton by an average of about 27 points. That's about 39 points better than he did overall in these states, which he lost by an average of 12 points.

You'll also notice that the margin Sanders won by among independents (29 points) was about equal to his win among those under 40 (27 points). That shouldn't be too surprising because, according to the Pew Research Center , millennials (who make up the vast majority of those voters under 40) are the most likely part of the Democratic electorate to identify as independents.

It's not difficult to imagine an under-50 former member of Congress who spent just three terms in the House like O'Rourke being able to sell himself to young independent voters.

We can get a little deeper into the weeds and look at multiple factors at a time to see what best accounted for support for Sanders during the 2016 primary season. An academic study known Cooperative Congressional Election Survey (CCES) allows us to do that well. It confirms what the entrance and exit polls suggest.

Among those under the age of 40, identifying as a Democrat or independent was nearly three times more important in explaining a person's 2016 primary vote choice than where they placed on the left-right spectrum. That is, young voter identifying as an independent was far more telling of their vote than how liberal they were.

Looking at all Democratic primary voters, I examined a person's age, race, ideology and whether they identified as a Democrat or independent in trying to explain vote choice. Ideology was the least important of these four variables in 2016. Age and party identification were by far the two most important.

In 2020, the question is whether Sanders can continue to appeal to independent-leaning young voters. With a load of young candidates with limited Washington experience likely to run, Sanders may find it more difficult to appeal to young voters the way he did in 2016.