Why is public opinion important in a democracy?

I would answer just slightly differently from the previous
post.  The people are not, in my mind, the ultimate source of power, but it is important
to maintain that illusioni.  Ideally of course, in a democracy the people vote to decide
their future, but in the case of a representative democracy, like ours with its party
system dominated by just two parties, public opinion is more important when it comes to
the perception of things rather than actually informing policy decisions,
etc.

So political parties must maintain a certain level of
positive public opinion but it can be relatively well manipulated through the media and
other sources and used to maintain the status quo and allow the representatives to make
decisions based on their interests.

Around election time it
becomes vital of course, but again, manipulation through the media and various other
methods counts for more than any kind of referenda or real reference to public
opinion.