He's not the first.
https://www.politico.com/news/2024/04/08/israel-gaza-war-elizabeth-warren-00151120
And Van Hollen and Merkley called it ethnic cleansing.
https://www.theguardian.com/us-news/2025/sep/11/democratic-senators-gaza-ethnic-cleansing
The direct proactive statement proclaiming it a "genocide" rather than answering a question or using "ethnic cleansing" is significant step though.