When you hear it being said that the govt. has a deficit that means that money is being taken from somewhere to finance the said short fall.
When a president starts a war that's costing America something like 4 billion dollars a week (it's some obsscene figure like this), it's not a coincidence that the deficit is that humongous. And when you think about what that money could be doing for the American economy other than financing the deficit, then you'll know the president's actions have a lot to affect the economy. A lot.
So he doesn't set the interest rates? Fine, you think the Fed Reserve just syas abracadabra to decide the rates? They factor in a lot of factors, and these are factors that are very directly affected by the govt and the president of the day.
It is not for nothing the presidential election is such a bi deal.