Ra'am MK Mazen Ghnaim voted against a law applying Israeli legal measures in Judea and Samaria.
If the law fails to pass, it may mean the end for the current coalition government.
After Ghnaim's vote, MK Nir Orbach of Yamina said the "experiment" with Ra'am had "failed."
"We tried but you have proven us wrong," he was quoted as saying.