Rep. Edward MarkeyEdward (Ed) John MarkeyLawmakers prep for coming wave of self-driving cars Trump taps Hill veteran for White House environment job Dems unveil push to secure state voting systems MORE (D-Mass.) on Tuesday compared the Supreme Court's ruling in the Citizens United case to the Dred Scott cast. 

Markey, who is running for the Senate, said the 2010 decision that led to the creation of super-PACS should be repealed, just as the Dred Scott decision that affirmed the legality of slavery was repealed. 

"I want to go to the United States Senate in order to fight for a constitutional amendment to repeal Citizens United. The whole idea that the Koch brothers, that Karl Rove can say we're coming to Massachusetts, to any state of the union with undisclosed amounts of money is a pollution, which must be changed," Markey said in the speech, given in Pittsfield, Mass.

He went on to say that "the Constitution must be amended. The Dred Scott decision had to be repealed, we have to repeal Citizens United."

Republicans quickly pounced on Markey's comments, calling his comparison "ridiculous and offensive."

"Agree or disagree with federal campaign finance law, it's ridiculous and offensive to compare it to the horror of slavery," said National Republican Senatorial Committee Communications Director Brad Dayspring.

The Supreme Court ruled in the 1857 Dred Scott decision that both free and enslaved people of African descent were not citizens, and thus lacked the protection of the Constitution. The decision essentially protected slavery.

Dayspring used the comments to highlight Markey's long tenure in Congress, an attack Republicans are trying to make stick as evidence the Massachusetts lawmaker is out of touch with voters.

"Out-of-touch statements like these simply reinforce the fact that Ed MarkeyEdward (Ed) John MarkeyLawmakers prep for coming wave of self-driving cars Trump taps Hill veteran for White House environment job Dems unveil push to secure state voting systems MORE has been hibernating in Washington back rooms since Carlton Fisk was behind the plate for the Red Sox and disco ruled the world. If Ed Markey really believes this to be the modern equivalent to slavery, he'll quickly return the millions that he's raised from PACs over his career."

Markey has, over the course of his career, received nearly $2.7 million in contributions from PACs.

But he's been an opponent of Citizens United and called for an agreement between candidates in the upcoming special election for the seat vacated by Secretary of State John KerryJohn Forbes KerryShould President Trump, like President Obama, forsake human rights in pursuit of the deal with a tyrant? GOP Senate report says Obama officials gave Iran access to US financial system Democrats conflicted over how hard to hit Trump on Iran MORE (D) to bar outside money from the race.

His Democratic primary opponent, Rep. Stephen Lynch, has agreed to that proposed pledge, but no Republican contender has yet agreed to the conditions.

In a statement issued to The Hill, Lynch said that while he supports, like Markey, a constitutional amendment to eliminate Citizens United, he did not agree with Markey's analogy.

“I don’t think it’s right to compare Citizens United to the Dred Scott decision. Dred Scott kept an entire race of people in bondage and perpetuated the horror of slavery in America.  I’m not sure there are any Supreme Court decisions that rival Dred Scott in infamy," he said.

Markey's campaign sought to refocus attention on the influence of outside money on politics in its response to the controversy.

"The Citizens United ruling is poisoning the democratic process in America, and that is completely wrong. Karl Rove, the Koch Brothers and anonymous special interests have no place in our elections and no place in this Senate race. I repeat my challenge to all Republican candidates to accept the people's pledge to keep this toxic outside spending and negative advertising out of the Massachusetts Senate race," Markey said in an email to The Hill.