If the prevailing media narrative is to be believed, as we head into next Tuesday's crucial New York state primary, Vermont Sen. Bernie Sanders – by virtue of winning seven of the last eight Democratic nominating contests – has gained crucial momentum, while Hillary Clinton has seen her earlier momentum slip away.

But is that true?

It's certainly a narrative that Sanders and his supporters have tried to popularize in their recent public comments. As Sanders told George Stephanopoulos this past Sunday on "This Week": "In the last three and a half weeks, we have reduced [Clinton's] margin by a third. ... We believe that we have the momentum. We believe that the polling is showing that we're closing the gap. Actually, as you may have noticed, of the last three national polls out there, we have defeated Secretary Clinton in two of them. So there's no question I think the momentum is with us."

In truth, the answer depends in part on what one means by momentum, which turns out to be a much-touted but often poorly defined concept. When pundits talk about momentum, they usually refer to one of two possibilities. The first refers to the winnowing of candidates, as typically happens early in the nominating process. When this occurs, it can appear that the remaining candidates gain "momentum" by virtue of picking up some of the departed candidates' support. There is evidence indicating this type of momentum does occur. However, that's not the type of momentum that pundits are referencing now, more than halfway through the fight for the Democratic nomination. Bernie's recent victories haven't driven anyone from the race.

There is a second type of momentum, however, one more consistent with how the term is being used in the current media narrative. It is the belief that a succession of electoral victories can increase the probability that the winning candidate will do better in subsequent contests simply by virtue of those previous wins. Under this scenario, winning begets more winning – the more wins, the greater the subsequent momentum – and losing has the opposite effect. When pressed to clarify how this type of momentum operates, proponents explain that winning leads to increased campaign contributions and more volunteers – resources that ultimately translate into more votes, and thus more wins. For those making this momentum-as-bandwagon argument, Bernie's current winning streak is clear proof that his momentum is very real – each victory during the last three-and-a-half weeks made it more likely that he would win the next contest. For this reason, Sanders and his supporters believe he is poised to do very well in next Tuesday's New York primary.

There's only one problem with this scenario. There's just not much evidence that momentum of this type exists, at least not in the recent context of Sanders' victories. Instead, the likelier explanation for Sanders' recent success (as I noted in my recent Professor Pundits contribution) is that the Democrats have held a string of contests on terrain that was particularly favorable to Sanders. Demographics, and not momentum, has been the key to his success.

It's no secret that Sanders does best in caucus states dominated by more ideologically motivated participants and in states with low minority populations. As it turns out, six of Sanders' last seven victories came in largely white caucus states. (Hawaii, a caucus state, was a demographic exception.) In fact, 11 of his 15 victories to date have come in caucus states. (He almost gained a 12th victory in the Iowa caucus, where he finished a close second to Clinton.) On the other hand, she has won 16 of the 21 primaries held so far. Indeed, if one constructs a regression equation to explain Sanders' vote share, the two biggest predictors are whether it is a caucus state and whether it had a large proportion of white, liberal voters. By this standard, one might argue he actually underperformed expectations in Wyoming, a largely white, caucus state, where he won "only" about 56 percent of the vote, less than he earned in several similar nearby states. More importantly, he split the 14 Wyoming delegates evenly with Clinton. That's not exactly the "momentum" he needs.

This is not to say that momentum is a completely meaningless concept. There is some evidence that voters' choices in the primaries are influenced in part by perceptions regarding how likely it is that the candidate is going to be elected. If a candidate can clear a certain threshold of perceived electoral viability, her chances of gaining additional votes increase.

But this is precisely where the Sanders' momentum argument works against itself. Because Sanders' recent victories have come predominantly in smaller caucus states and because of the Democratic Party's proportional delegate allocation rules, Sanders' winning streak hasn't substantially cut into Clinton's delegate lead, at least not nearly enough to alter the perception that she remains the clear favorite to win the nomination. Since March 22, when Sanders' current win streak began, he has gained a net of only 70 pledged delegates on Clinton and still trails her by more than 250 pledged delegates. Her lead expands to more than 700 if one includes superdelegates. Moreover, Clinton can more than wipe out Sanders' recent gains with a strong showing in her home state of New York next Tuesday, where there are 247 pledged delegates at stake.

This failure to clear the viability threshold has two unfortunate consequences for Sanders. First, despite his claims to the contrary, his recent victories provide little reason for Clinton's superdelegate supporters to change their minds and back Bernie. Second, to the extent that perceptions of electoral viability matter to prospective voters in upcoming states, it is Clinton and not Sanders who is most likely to benefit. She is the perceived front-runner, and thus she is more likely to gain the support of voters who want to back the race favorite. And those perceptions of viability are not likely to change in the foreseeable future, as the Democratic race returns to terrain, in the form of larger, more demographically diverse primary states, likely more favorable to Clinton, including Pennsylvania, New Jersey and California. On the other hand, and unfortunately for Sanders, only one of the remaining 16 Democratic contests is a caucus state.