While anything is technically possible, I can't imagine a world where national mood the Republicans pick up the Senate, but NC elects a Democrat. Something has got to give.
What's so implausible about this scenario where the Republicans hit 51 Senate seats?
That scenario is basically Democrats winning the competitive races in the 2008 Obama states and Republican winning in the 2008 McCain states. That's why I'm a pessimist about the Democrats keeping the Senate. Republicans need to lose a competitive races in either Arkansas, Louisiana, Georgia, Alaska, Kansas or South Dakota, while Democrats win all the competitive races in swing states like North Carolina, Iowa and Colorado.