Reid, D-Nev., described Charles and David Koch as "un-American," the Los Angeles Times reported.
"It's time that the American people spoke out against this terrible dishonesty of these two brothers, who are about as un-American as anyone that I can imagine," Reid said in his second speech on the subject during the day.
Charles and David Koch, two of the four sons of Fred Koch, are top executives at Koch Industries and have become prominent recently for giving large sums to conservative candidates and political groups like Americans for Prosperity. While most of Charles Koch's giving promotes free-market views, his brother has also made large donations to medical research, the arts and education.
In his first speech, Reid described a new ad campaign featuring people who say they have been harmed by the Affordable Care Act as "absolutely false." In his second, he said not all the ads are false, just the "vast vast majority of them."
A Koch spokesman accused Reid of going after a cancer victim, referring to an ad featuring a Michigan woman who has leukemia and says she lost her insurance because of Obamacare. She blames Rep. Gary Peters, a Democrat now running for the Senate in Michigan.
"We believe it is disgraceful that Sen. Reid and his fellow Democrats are attacking a cancer victim as part of their campaign against Charles Koch and David Koch," Philip Ellender, president of Koch Companies Public Sector, LLC, Government and Public Affairs, said in a statement.