Actually it was the absolute lowest. Only two challengers have gone below 40 against him and only one, O'Donnell, went below 37.5.
In her 2008 Senate campaign, O'Donnell ran as well as any Republican in Delaware. McCain only got 37% of the vote in Delaware.
It would seem that she did quite well in Del. for a Republican in the 2008 election.
Again, the issue was since she was the GOP nominee in 2008 she must not have been considered too crazy to win then.
Ofcourse, the difference between then and now is that the Party elites had Castle lined up for the office.