A win in Nevada would mean that Sanders basically tied or won every single primary so far. Remember, no candidate has won either party's nomination without coming in first or second in New Hampshire in the modern primary era.
Now, there is an argument to be made that Sanders may have a ceiling of support nationally. Although he was up to 31% in the Marist poll, his 25% in the average polls looks an awful bit like the 26% he earned in Iowa caucuses
and New Hampshire primary
. Late deciders in both of those contests went overwhelmingly for the other candidates suggesting undecided voters may flow against Sanders nationally.
Still, Sanders is very well liked nationwide. His favorable rating among potential Democratic primary voters was 76% in a Quinnipiac University poll taken after Iowa voted,
and Sanders was the second choice of 11% of voters. His combined first and second choice support was higher than it was for any other candidate in the poll. In other words, there is clearly the opportunity for Sanders to continue to gain ground.
But even if Sanders doesn't pick up any additional support, it's important to note that he is by far in the best position to pick up delegates in the upcoming contests.
Sanders is hitting 15% of the vote in most state polls
. By reaching 15%, he's putting himself in a position to pick up delegates in most states. No other candidate can say that as confidently at this point.
Sanders is benefiting from the fact that his coalition is racially diverse. Across an average of recent national polls, he's polling at 20% or greater among African American, Hispanic and white voters. Aside from perhaps Florida (with its very old voter base), it's difficult to find a state where Sanders isn't at least somewhat competitive.
All of this means that Sanders has by far the best chance of any individual candidate of winning a plurality of delegates overall. Unless someone else emerges from the pack (say Bloomberg), the other Democrats best shot to stop Sanders may be a contested convention (i.e. no one reaching 50% of the delegates). And a scenario in which Sanders wins the most delegates but loses at the convention could tear the Democratic Party apart.