Binyamin Netanyahu has announced he is to challenge Prime Minister Ariel Sharon for the leadership of Israel's ruling Likud Party.
Netanyahu has bitterly denouced Sharon's Gaza withdrawal
Mr Netanyahu told a news conference in Tel Aviv that Mr Sharon's actions were destroying their party.
"The Likud needs a leader who can unite the ranks... and take it to victory [in next year's election]," he said.
The former prime minister resigned as finance minister earlier this month in protest at the Gaza Strip withdrawal.
"Sharon gave and gave and gave some more, the Palestinians got more and more and more," he told reporters in Tel Aviv.
"What did we get in return? The answer is: nothing, nothing and nothing. The opposite.
"The Gaza Strip is turning into a base for Hamas, Hezbollah and al-Qaeda and the terrorists' motivation is growing stronger."
Mr Netanyahu and his followers are looking to punish Mr Sharon over the disengagement plan, reports the BBC's Matthew Price in Jerusalem.
He says current polls show that Mr Netanyahu would win a leadership contest if it was held today.
But as far as the country is concerned, our correspondent says, he still remains a less popular politician than Mr Sharon.
Mr Netanyahu's anouncement comes after Mr Sharon laid into his political rival in an interview with Israeli TV on Monday.
The prime minister said Mr Netanyahu was an "uptight and pressurable individual who panics and loses his wits".
He was adamant that he would still be head of Likud at the general election, scheduled for next February.
"I was elected by the Likud and I will run on behalf of the Likud, and I believe that I will form the next government," he said.
Likud's Central Committee is due to meet on 25 September to decide on a date for the start of the leadership vote, expected to be in November.