president will follow-up this tweet with an official endorsement of senator jeff flake's challenger in the primary. cnn asked flake about that yesterday. >> how about the president calling you a non-factor in the senate and toxic? >> don't -- don't worry about it at all. go ahead and doing my job. >> reporter: is it the president's place to endorse a primary challenger? particularly somebody against his own party? >> that's not my realm. i'm running my own campaign. it's going well and what the president does is his prerogative. >> and yet another subplot we're watching as trump heads west, the possibility he'll harden former maricopa county sheriff joe arpaio, found guilty of criminal contempt refusing to follow a judge's order barring from racially profiling latinas. trump hinted last week we may let him off the hook. arpaio has not said he's been