Mitchell was not for open borders. He was also a former mayor of Tempe. He was a very strong candidate in a district with changing demographics. It also doesn't hurt when the Dem candidate says he supports the McCain-Bush plan on comprehensive immigration reform. FYI: Hayworth voted against the House bill.
Hayworth was still the national poster boy for the border issue. Other issues were at play certainly, but if shutting down the border were the preferred position, he should have won in a landslide.
The fact that his opponent favored comprehensive reform is quite telling. However, support for any Dem congressman regardless of his particular stand, was a vote for opening the borders.
I guess a lot of folks refused to realize that.