DeMint, who appeared on ABC's "This Week" Sunday, cited a study that his organization, the Heritage Foundation, will soon release.
"The study you'll see from Heritage this week presents the staggering costs of another amnesty in our country and the detrimental effects, long-term, that that will happen," he said. "There's no reason we can't begin to fix our immigration system so that we won't make this problem worse. But the bill being that's being presented is unfair to those who came here legally."
DeMint also accused President Barack Obama and members of Congress of skewing the facts by saying the reform will increase economic growth.
"Well, the Congressional Budget Office said Obamacare wouldn't cost us anything. They're basically puppets of the Congress and the assumptions they put in the bill. Heritage is [the] only organization that has done an analysis on the cost," DeMint said.
DeMint also said he opposes the bill because it takes away opportunities for those who come to the country legally.
In response, former Mexico Gov. Bill Richardson said the bill is necessary, and that the immigrants will contribute to the economy.
"My beef right now is the path to citizenship. I think it's 13 years. It's excessive," he said. "You have to jump through a lot of hoops."