1. Apathy on our part.
If those of us who value freedom and Liberty are silent, or seen as unconcerned, it has a good chance of passing. The Million Sucker Moms, VPC, and media have a loud, if not numerous, voice, and it can and has drowned out the facts in the past. If those on our side could get even one-quarter of our number to a march on Washington, we'd never face this crap again.
2. A mass-shooting incident between now and then.
Regardless of firearm involved, such a massacre would produce a wave of emotionalism and hysteria that the Republicans would be unable to stand in the face of without MASSIVE backup from ALL of us. This close to an election, such an emotional whomp would be near-overwhelming.
By no means did I mean it was time to back off and let off the pressure. The circumstances you cite could make a difference.
All those who legally carry should be extra vigilant for such an incident as well as terrorists attacks between now and the election.