jumping over fiscal cliff probably is not as bad as it sounds. This is because even if the budget deal does not occur before December 31, Congress can and will act to change the scheduled laws retroactively to January 1 after the deadline.
But a divided congress will almost definitely lead to another downgrade on US credit rating. And that will hurt investors' confidence even more. Economy works like a ponzi scheme and the confidence in the future is ultimately important to the game.