Why is it illegal for Obama to help pay DNC's debt?

The DNC owes $15 million to....someone....for putting out a last-minute campaigning surge in the final days of the election. I've read that it's illegal for Obama to help pay for it out of his 600 million dollar coffers, but can't find any reasons why.

I'll keep searching for an answer, and update my post when I find it. In the meantime, if you're reading this blog and you know the answer, please feel free to comment!