Thomas Sowell knows why politicians on both sides of the aisle are so upset that the Supreme Court overturned limits on campaign contributions—because such restrictions hurt political challengers and protect incumbents. I think he hit the nail on the head:
The recent Supreme Court decision over-ruling some Federal Election Commission restrictions on political campaign contributions has provoked angry reactions on the left. That is what often happens whenever the High Court rules that the First Amendment means what it says—free speech for everybody.
Opponents say killing these limits will lead to electoral corruption. What exactly do they mean? President Obama cries that removing restrictions “will open the floodgates for special interests.” Sowell explains, “Those unfamiliar with political rhetoric may not know that ‘special interests’ mean people who support your opponents":