He thinks this bill will endanger the Iran nuclear deal from 2015. From Sanders' website:

"I am strongly supportive of the sanctions on Russia included in this bill. It is unacceptable for Russia to interfere in our elections here in the United States, or anywhere around the world. There must be consequences for such actions. I also have deep concerns about the policies and activities of the Iranian government, especially their support for the brutal Assad regime in Syria. I have voted for sanctions on Iran in the past, and I believe sanctions were an important tool for bringing Iran to the negotiating table. But I believe that these new sanctions could endanger the very important nuclear agreement that was signed between the United States, its partners and Iran in 2015. That is not a risk worth taking, particularly at a time of heightened tension between Iran and Saudi Arabia and its allies. I think the United States must play a more even-handed role in the Middle East, and find ways to address not only Iran's activities, but also Saudi Arabia's decades-long support for radical extremism.