See here. The article makes a lot of good points about Trump and his supporters, including the ones on talk radio, like Limbaugh. I also recommend listening to the audio clip he links featuring Mark Levin's comments on Trump in 2011. Notice how negative Levin was about Trump at that point. And notice that what he criticizes Trump for then is applicable today as well (his history of liberalism, his self-contradictions, his opportunism, etc.). Levin's comments about Rubio in that 2011 clip are ironic, given that Trump is now running against Rubio in this 2016 cycle.
I want to add a qualifier to Guy Benson's analysis, though. Limbaugh often hedges his bets, and he often does it in a dubious way. His approach toward Trump is an example of that. Limbaugh does sometimes acknowledge that Trump isn't a conservative, that some of Trump's opponents have better reasons for opposing him than the Trump critics Limbaugh usually focuses on, etc. So, Limbaugh could highlight his comments like the ones I just described in response to somebody like Benson. But Benson is right about how Limbaugh usually covers Trump. He has the general thrust right, even though Limbaugh can cite some occasional bet hedging to defend himself against that sort of criticism. He'll occasionally provide a more reasonable assessment of Trump, but he spends the large majority of his time doing what Benson rightly criticizes.