While the strong brand and loyal customer base make a full closure unlikely, the pool of investors who will be able to pull the struggling airline back on track is limited — and, now that Spirit has rejected a merger offer from Frontier ( FRON) for a second time, got even smaller.

After filing for bankruptcy protection and going back and forth with Frontier Airlines — again — over a potential acquisition, Spirit Airlines has a new plan.

Spirit Airlines on Thursday rejected Frontier Airlines' latest proposal for a merger, which would have helped bring the Florida-based carrier out of bankruptcy to join forces with its top competitor.