I can't believe people aren't getting this.
She's just like him. She goes after her own party, she fights corruption, she's a gritty maverick, she's tough.. She's a femaile Teddy Roosevelt, or a female 2000 John McCain.
In addition, she's incredibly likable, and she's a woman, and her personal story is pretty interesting, and she looks good on camera. She simultaneously stabilizes the base, and emphasizes mccain as a guy that doesn't follow the party line. Everybody in the party wanted him to pick Romney. He said "Fuck you guys"
I'd personaly say he probably picked her because he admires her record, just as much as her ability to steal Hilary voters. It's smart politicaly, and her record matches and doubles down on McCains personal story.