My guess is around 75%. Polls show most Israelis support "negotiations" with Hamas, handing over at least part of Judea/Samaria, and at least somewhat supportive of the disastrous handover of Gaza three years ago. However, many of these "average Israeli leftists" are ignorant sheep who believe what they do due to brainwashing and not knowing an alternative exists.
My guess as to how many Israelis are hardcore, incorrigible leftists is probably more like 40%.