Sen. Bernie SandersBernard (Bernie) SandersSchumer: Franken should resign Franken resignation could upend Minnesota races Avalanche of Democratic senators say Franken should resign MORE (I-Vt.) said billionaire brothers David and Charles Koch are trying to buy the Senate.

“The Koch brothers are trying to buy the Senate,” Sanders said Wednesday. “This is not what democracy is supposed to be about. Billionaires should not be allowed to spend hundreds of millions of dollars electing candidates who represent the wealthy and the powerful.”

Sanders called for the passage of a constitutional amendment that would allow Congress to reverse the Supreme Court’s 2010 Citizens United decision, which struck down campaign spending limitations. Senate Republicans blocked that amendment last month.

The Supreme Court decided that campaign donations are protected as free speech under the First Amendment, but Sanders said the ruling has allowed America to become an oligarchy, where only a handful of wealthy people control everything.

“We are not living in a democracy when 60 percent of Americans are not voting, while billionaires like the Koch brothers are spending hundreds of millions to buy the United States Senate,” Sanders said. “That's called oligarchy, not democracy.”

The Koch brothers have spent millions supporting conservative politicians. Republicans need to flip six seats to take control of the Senate and several vulnerable Democrats are up for reelection in November.