Sanders praised Clooney's acknowledgment in an interview aired Sunday on NBC's "Meet the Press" that "it's ridiculous that we should have this kind of money in politics."
"They're right to protest," Clooney said of Sanders backers who protested his fundraiser for Clinton. "They're absolutely right. It's an obscene amount of money."
Asked in an interview with CNN's Dana Bash on "State of the Union" Sunday if Clooney is backing the wrong horse, Sanders said, "Well, I think he is."
"He is honest enough to say that there is something wrong when few people -- in this case, wealthy individuals, but in other instances for the secretary, it is Wall Street and powerful special interests -- who are able to contribute unbelievably large sums of money," Sanders said. "That is not what democracy is about. That's a movement toward oligarchy."
He added: "This is the issue of American politics today. Do we have a government that represents all of us, or only the 1%? And you're not going to have a government that represents all of us so long as you have candidates like Secretary Clinton being dependent on big-money interests."
A group of around 100 Sanders supporters showered Clinton's motorcade in 1,000 single-dollar bills as the 2016 Democratic presidential candidate drove to Clooney's fundraiser on Saturday. To co-host the event, a couple was asked to donate $353,400 to the fund. Single tickets cost $33,400.
Sanders' criticism of special-interest money in politics is at the core of his campaign, even though he has outraised Clinton in recent months on the strength of a powerful online, small-dollar fundraising operation.
On Friday, he released his tax returns from last year. On Sunday, Sanders said more years are coming, "probably" this week.
Sanders, who just returned from a brief trip to the Vatican where he briefly met Pope Francis, denied any suggestion that the Pope was embracing his politics.
Showed a clip of Vice President Joe Biden in an interview with CNBC saying any insinuation that Pope Francis was endorsing Sanders' political proposals was off-base, Sanders agreed.
"I love Joe, and no one is suggesting that the Pope was embracing my policies," he said. "I was very proud to have been invited by the Vatican to an important conference dealing with morality in economics, and in my view that is exactly the issue of the day."
Sanders added: "We have got to create an economics that is based on morality, dealing with the needs of working families and the elderly and the children and the sick and the poor, rather than an economy that is based on greed and the needs of big corporations."
Sanders also talked guns on Sunday. He said he supports the assault weapons ban backed by the parents of the victims of the Sandy Hook Elementary School massacre.
But he still doesn't want the owners of gun stores who sell small firearms legally to people who later use them in crimes to face legal liability for those crimes, he said.