I believe the rule of thumb for incumbents is if they cannot poll at 50%, they are in big trouble.
More like 45% or under. 48% is close enough to 50% that if the opponent is truly at 45% like this says, most of the undecideds can still break for the challenger but he is still close enough to 50% to get there with far fewer of the undecideds going his way. Now if it was 45% to 42%, he would be in much bigger trouble, or even 48% to 47%. Of course it also depends on how high the challenger’s negatives are...and the reason he is even still in this is because the SuperPACs swooped in months ago and have been running blistering negative ads against Rosen non-stop.