Sen. John McCain (R-Ariz.) will handily defeat primary challenger J.D. Hayworth unless there’s a dramatic shift in fortune between now and August, according to a new poll.

McCain gets 64 percent of the test vote compared to only 19 percent for Hayworth in a new Rocky Mountain Poll by the Phoenix-based Behavior Research Center. Navy veteran Jim Deakin (R), meanwhile, gets 5 percent, with 12 percent of respondents undecided.

Hayworth’s appeal to Republican primary voters has dropped steadily since April, with his support shrinking from 28 percent to only 19 percent. McCain’s TV ads may have played a part in that drop. The senator has run three TV spots since June 23 slamming the former congressman for being a “huckster.”

With early voting set to start July 29, Hayworth won’t have much time to turn things around. His best opportunity will be this weekend, when the three Republicans debate on Friday in Phoenix and on Saturday in Tucson.

But even a strong debate performance may not be enough to salvage Hayworth’s challenge.

Independents who plan to vote in the GOP Aug. 24 primary overwhelmingly back McCain over Hayworth.

McCain’s only concern is his soft job-approval rating, which could spell trouble in the general election. In the survey 43 percent of Republicans rate McCain’s job performance as “only fair” or as “poor.”

In the overall electorate, 36 percent have a favorable view of McCain, while 22 percent have a poor view.