Running against McCain was a follish decision of Kirkpatrick.
I think the real reason she entered initially, though she'd never admit it, was because she was afraid that the supreme court was going to take away the state's independent redistricting commission, giving the legislature the ability to give her a district too republican for her to get re-elected in. Of course, she had to jump in before the decision was made to avoid being attacked over jumping races for that reason.
When SCOTUS decided to keep the independent commission in place, Kirkpatrick had the choice between switching back to running for house re-election, which would risk the whole incident being made into a significant scandal, or staying in the senate race and hoping that McCain would either get primaried or run an inept campaign. She chose the latter, but it didn't work out.