Why would the Republicans even target a seat they didn't win in 2010?
That's sort of like asking "why would the Democrats target Chris Shays in 2008 when they couldn't even beat him in 2006?" Just because someone survives a wave (even a 2010-level wave), doesn't mean they're invincible. That said, Chandler's probably going to win by 10-15 points.
This. Similar story with Larry Kissell. He came within a few hundred votes of ousting Robin Hayes in 2006; in 2008 he won by 10 points. That said, that probably won't be the case here given Chandler's family popularity and the Democratic lean of this area in local elections.
Slight difference: Chandler was made - somewhat marginally, though perhaps less so at the state and congressional than the presidential level - more Democratic in the intervening redistricting, the enacted compromise decried by Lexington Republicans as "the Ben Chandler Lifetime Employment Act". Which is hilarious hyperbole, of course, but still hyperbole rooted in actual fact.