I think, assuming Obama wouldn't be stupid enough to make her his running mate, that she would stay out of the election. If the Republican nominee won, she'd fight him in the Senate. If Obama won, she would propose her ideas in the Senate. Whenever one of her ideas would be signed into law by President Obama, she would take credit. If her idea later went badly (highly likely), she would blame the Obama Administration for failing to implement it properly.
I think if Hillbag wins the nomination and fails to include Bin Laden Obama, that could fracture the black base that dems now hold.