You're undoubtedly right about where the blame lies. I've seen it myself in an important race, where the national GOP wrote off a candidate because he was against a popular Democrat incumbent, only to see the GOP candidate lose by a hair. They regret it now, I'm sure!
That race was important, but not as important as this Senate seat in Illinois. If what you say is true, it's really boneheaded they didn't take it seriously before now. Then again, often they don’t step in until the general election.
And indeed, I've seen no evidence they're taking it seriously now either.