Why would it be extremely difficult to effectively limit campaign spending?If campaign spending limits were effective, who would be hurt more those...

One reason that it would be very difficult to limit
campaign spending is that the Supreme Court has pretty much said that campaign spending
cannot be limited in any serious way.  There can be limits on what is given to
candidates, but you essentially can't limit what people spend on their own (not
coordinating with the candidate).

In general, unlimited
spending helps incumbents.  People who spend money on campaigns generally want something
in return (at the very least, they want access to power).  The incumbents are the ones
who have the power and are more likely to keep the power than the challengers are to win
the power.  So people will generally spend more money to help incumbents than to help
first time challengers.