Polls I've seen have Arpiao in third place but not by that much - 6 points or so. If it looks like Arpaio is Trump's preferred candidate then what will that do to those results? And the way Pence was talking sure makes it look like a preference for Arpaio.
I think President Trump would prefer Kelli Ward as candidate. I went to a fund raiser for Ward and Sebastian Gorka was the featured speaker. Also rand Paul, Hannity, Levin, and Ingraham have endorsed her for Senator.