I found this
online. It appears that Mack is struggling to raise enough money to compete with Nelson. He also isn't generating enthusiasm amongst Republicans, which may explain the drop into the 30's.
The comment at the bottom about Romney and Mack having their fates connected may refer to more then just chances of winning. They both are rich children of politicians and they both have trouble connecting with people.
If he can kick it into gear campaign wise, he might still have a chance provided Mittens wins Florida in November.