Tesla Motors (NASDAQ:TSLA) is starting to rev up again, and that should translate into pain for the growing number of naysayers willing to put their money where their bearish thoughts are.

The stock exchanges released  their latest short interest data on Christmas Eve, and the number of folks betting against Tesla continues to inch higher. There were 30.3 million shares of the electric-car maker sold short as of Dec. 13, far more than the 18.5 million shares shorted when interest bottomed out in mid-July. 

We're closing in on the 32.3 million shares held short on March 15, and it's important to remember where the stock was at the time. Shares of Tesla closed at $35.29 that day. Nine months later we find ourselves with a market darling that has more than quadrupled despite the pullback that investors experienced earlier this quarter.

Tesla's been debunking skeptics as it expands overseas to grow sales of its critically acclaimed yet pricey Model S sedan. This week it gained more momentum after the National Highway Traffic Safety Administration stuck its five-star rating for Tesla's signature vehicle. Some feared that recent news of fires as a result of road debris penetrating the battery pack in a couple of cars would ding its safety rating. It didn't.

The spike in short interest may be seen by worrywarts as a sign to stay away, but we already saw how it should have been a dinner bell for bulls during the stock's springtime peak in bets placed against Tesla. We've now seen short interest move higher for four consecutive reporting periods, and that's not exactly a bearish sign.

Date Short Interest
10/15/13 20.8 million
10/31/13 21.4 million
11/15/13 25.4 million
11/29/13 29.2 million
12/13/13 30.3 million

Source: Nasdaq

What does this table tell us? On the surface it tells us skepticism grows about either Tesla's viability or its valuation. One can argue that $19 billion is a pretty steep sticker price for a company delivering less than 2,000 cars a month. But that does a disservice to how Tesla's pulled off three quarters of better-than-expected profitability this year, and how its prospects for international expansion, next year's Model X crossover SUV, and the more economical model to follow require a broader horizon in weighing what Tesla's worth. The last time that short interest got this high the bears paid dearly. Tesla's unlikely to quadruple over the next nine months. Tesla at $80 billion would certainly be a stretch. But now that we're back to more than 30 million shares sold short, it sets the stage for a short squeeze when positive catalysts force the naysayers to cut their losses and cover their positions. 

Tesla's not cheap, but that doesn't mean that it won't be less cheap in the near future.

Longtime Fool contributor Rick Munarriz has no position in any stocks mentioned. The Motley Fool recommends Tesla Motors. The Motley Fool owns shares of Tesla Motors. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.