Corruption - A Changing Definition
Buckley v. Valeo (1976)
The Supreme Court ruled that corruption or the appearance of corruption is a justification for limits on free speech rights in connection with political contributions. Two further decisions in 1986 and 1990 ruled that distortion of the political process can occur through undue influence by large donors on candidates and officeholders.
Citizens United v. the FEC (2010)
The court narrowed the definition of corruption in the context of campaign finance regulation to a quid pro quo exchange. Quid pro quo is a Latin phrase that literally means "this for that" + namely, bribery - an explicit agreement by a candidate or elected official to perform a specific act in exchange for something of value. In the current context of political campaign finance, it refers to the only kind of corruption that justifies limits on First Amendment rights.
Narrowing the definition
Narrowing the definition of corruption fails to recognize distortion of the political process when large donors can spend unlimited money which can drown out the speech of ordinary people. Distortion can occur when the actions of elected officials who are responsible for representing the interests of their constituents conflict with the subtle influence of favored access that may be granted to large donors and their lobbyists by elected officials and their staffs. Another effect of large campaign contributions can be to cause citizens to feel that the political process does not represent them and can discourage them from participating in the political process.
Hence the Court rejects the notion of big money corrupting the election process or diminishing political equality, in favor of allowing citizens, not government, to decide what speech they will hear.