Portman said the limits violate First Amendment rights to free speech.
"Look, the First Amendment has always been the bedrock of our democracy," Portman said, "and what the Supreme Court said is this law shouldn't infringe on the First Amendment by telling them how many candidates they can support."
That 5-4 decision struck down limits to total contributions made during an election cycle. However, donors are still limited to $2,600 per candidate per election.
Republican Sen. John McCain -- who championed campaign finance reform -- said in a statement that the ruling dismantled efforts to limit the "undue influence of special interests."