on the bubble

Short definition from Wiktionary

Prepositional phrase

1. 1
   Holding the last qualifying position with qualification still in progress, and thus liable to lose that position.
2. 2
   Having qualification for an event depend on the upcoming performances of other competitors; hence, at risk of being eliminated, and reversely, being close to qualification; on the verge.
3. 3
   Uncertain of success.
4. 4
   Used other than figuratively or idiomatically: see on, bubble.