Any particular reason why Cahill has recently fallen from 30% to single digits ?
He has been Teabaggin'.
Because he decided to track hard right, the GOP was worried that Cahill would peel off Baker voters to the extend where Cahill would play spoiler. To prevent that, the RGA spent big bucks to scuttle Cahill's support amongst Republicans early and ran some scathing negative ads.
It worked. His numbers amongst Democrats were already bad, so now he's got absolutely no support save from a few stragglers.