It should still be possible. It depends on your attainment in the exam. It's done on averages. However, the ISA and exam weighting are't to my knowledge 50:50, so the exam is still your best bet.
Look at it this way:
If you get a C in the exam, you're almost definitely in for a C.
If you get higher than your ISA grade in the exam, the overall mark will be between the two and vice versa if your ISA is higher than the exam. I wouldn't be too disheartened though - GCSE history I achieved a D in the exam but thanks to A* coursework, I got a B.
EDIT: Your exam board may have a UMS or mark calcualtor that allows you to put together marks accurately, like AQA's UMS calculator. I'd search for that - it'd give you a god idea of what to aim for.