If the Supremes rule against Obamacare, which I sincerely hope they do, then that will actually help Obama.
Since the majority of Americans are against Obamacare, if the Supremes rule in favor of it, then the Republicans and others can rail against Obama, and promise a vote for them will be a vote to kill it. It will be the last time those against it (which is a majority of folks) can do anything about it, and will lead to a more enthusiastic vote.
On the other hand, if the Supremes declare it Unconstitutional and strike it down, many voters will be satisfied with that ruling and will not be so fired up come November. Some who would vote in anger against it will not bother voting as it is no longer an issue. I still don’t see that as enough to get Obama over the edge, but with almost certain democrat voter fraud and extremely biased media help, who knows!
You're assuming all of ObamaCare would be struck down. That's unlikely. If the Supreme Court strikes down the Individual Mandate, but treats it as severable, then the Republicans/Conservatives/Romney will still be able to use what remains of ObamaCare as a campaign issue.