Joseph: Outlawing campaign contributions would, by that very fact, give rich candidates the upper hand. There would be no way for poorer candidates to compete in the publicity dept., which is a major factor in winning elections.
Also, regarding the constitutionality of McCain-Feingold, there is a strong argument to be made that it is curtailing our freedom of speech.
From Wikipedia on Campaign Finance Reform: Most opponents claim that CFR infringes on free speech and violates First Amendment rights. The argument is that the purpose of the free speech clause of the First Amendment is the guarantee that people have the right to publish their political views. Under this view, when the laws prohibit people from advocating for or against political candidates by restricting the content or the amount of political advertising, the laws are in conflict with the constitutional guarantee of freedom of political speech.
There also other criticism of CFR in McCain-Feingold: In addition, many opponents point out that campaign finance regulations are excessively complicated. This, they say, prevents ordinary citizens from participating in the election process (especially from running for office) and limits participation to a wealthy elite who can afford the legal apparatus necessary to run. In modern campaigns, legal and accounting expenses are significant percentage of the overall budget. Opponents also claim that excessively complicated rules discourage participation more generally by dissuading people from even attempting political work or activism.
Personally, I think John McCain’s own law limited his chances to win the 2008 election, unintentionally of course.