Members of Israel's governing Likud party have begun voting on Prime Minister Ariel Sharon's controversial Gaza pullout plan.
The conflict in Gaza claimed new victims on the eve of the poll
The proposal calls for Israeli troops and 7,500 Jewish settlers to leave the Gaza Strip.
Opposition to the plan is said to be strong among the 193,000 Likud members, and a No vote could trigger a crisis.
Meanwhile, two Palestinian gunmen killed five Jewish settlers in the Gaza Strip - a mother and four children.
Army Radio said the militants had opened fire on a vehicle, and detonated a bomb at an army post.
Both gunmen are also said to have been killed following the attack.
Mr Sharon told Israeli radio the "terrible assassination" was intended to "delay and disrupt" his disengagement plan.
But advisers to Mr Sharon indicated earlier that he would push for a Yes vote in parliament regardless of how the Likud members voted.
Mr Sharon made a last-minute appeal for support as he went into his weekly cabinet meeting.
He said the vote would be decisive for the future of Israel, its security and its economy.
The prime minister has warned that if his plan is rejected, it could bring down the government, says the BBC's David Chazan, in Jerusalem.
"He [Ariel Sharon] will not let go of this plan," one adviser told AP news agency on condition of anonymity.
The Israeli premier has portrayed his withdrawal plan as part of the peace process with the Palestinians but they have accused him of seeking to redraw borders without their consent.
Under the plan, most of the 400,000 settlers in the West Bank and East Jerusalem would stay put.
Israeli Defence Minister Shaul Mofaz went on radio on Saturday to urge a Yes vote.
"The Sunday vote is a historic chance for Likud members to decide the fate of a plan that is good for Israel," he said.
Sharon has been canvassing for support among party activists
"We cannot afford to miss this opportunity."
Mr Sharon has dismissed his opponents as "extreme right-wing" and warned that rejection of the plan could lead to early elections.
Opinion polls have shown that about 80% of Israelis as a whole back withdrawal from Gaza.
The No campaign has been strong with activists visiting tens of thousands of Likud members in their homes and thousands of Gaza settlers spending the weekend with relatives in Israel.
The settlers say that leaving Gaza would be seen as a victory for Palestinian militant groups like Hamas.
The BBC's David Chazan, reporting from Jerusalem, notes that this is an argument which resonates among many Israelis who remember with bitterness how Hezbollah guerrillas celebrated Israel's withdrawal from south Lebanon.