Your simple answer in one chart? Everyone but Bush and Walker:
In fact, Trump and Walker both seem to be benefiting from the demise of just about every other candidate in the race, while Santorum and Kasich seem to be making modest gains.
Interestingly enough, Cruz seems to be hovering in that middling pack of second-tier candidates, actually losing support in the wake of Trump’s rise.
…and Trump’s rise shows no sign of abating, even after his comments regarding McCain’s war service.