Why would it be extremely difficult to effectively limit campaign spending?
If campaign spending limits were effective, who would be hurt more those politicians already in office or those attempting to win an election for the first time?
In the end, campaign spending is an expression of the First Amendment right to political expression and voicing the will of an individual. When an individual or company gives to a campaign, it is a form of petitioning their own government about their particular beliefs and understandings. Seeing that corporations and people have been deemed as possessing similar legal rights, it is difficult to effectively limit the role of money in the political process because each individual/ corporation can use the First Amendment political expression as rationalization for what is being done. It is highly improbably that the presence of money will leave the political realm as it is a part of the process and helps drive a great deal of candidates' success. Candidates who have wealth and have amassed a great deal of financial security are more likely to experience political success than those who don't have any.
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.