It is widely assumed that politicians are routinely influenced by campaign contributions. That is usually why businesses and unions make such contributions, and it is why many people--including many members of Congress themselves--believe the campaign-finance system should be reformed. Yet criminal investigations or prosecutions are rare (and usually involve violations of the fund-raising limits rather than any quid pro quo). Isn't it illegal for an elected official to trade a vote or other official action for a campaign contribution?
Answer: If a senator were to write a letter saying, "Dear Big Donor: Give my campaign $1,000 and I will vote to renew the tax break for your industry," and if Big Donor were to donate $1,000, that would constitute illegal bribery. But anything short of that, in terms of evidence or context, is either not illegal or impossible to prosecute. For example, a campaign donation after the fact--"Thanks for voting yes, senator. Here's $1,000 for your re-election"--is perfectly legal, even though the connection between the donation and the vote is explicit. And of course in most cases there is no evidence of an explicit connection.
After the "Keating Five" scandal of the late 1980s (in which five senators, including John McCain, were accused of interfering with the investigation of a large donor's savings and loan), the Senate ostensibly tightened its own ethics rules to go beyond the legal requirements. "Rule 43" states that senators can--and should--assist citizens in dealing with Congress and other governmental agencies. But it bars them from doing so "on the basis of contributions or services, or promises of contributions or services, to the Member's political campaigns." In other words, a senator supposedly cannot treat a donor differently from any other constituent. Doing so could result in censure or expulsion. The Senate has not yet convicted any members of violating this rule.
NOTE: Many activities are wrong although they are not illegal. Timothy Noah sorts out the rights and wrongs in "Congressional Ethics for Idiots."