FACTBOX - Israel leadership vote and what follows

(Reuters) - Israel's ruling Kadima party votes in three weeks for a successor to Prime Minister Ehud Olmert, who has said he will step down because of corruption investigations.

The following are mechanics of the primary and scenarios for how the winner may -- or may not -- become prime minister.

* On September 17, Kadima's 70,000 members head to polling stations at 22 party offices to cast ballots.

* For now there are four candidates, though some may yet drop out: Foreign Minister Tzipi Livni, Deputy Prime Minister Shaul Mofaz, Internal Security Minister Avi Dichter and Tourism Minister Meir Sheetrit.

* Polls indicate Livni will beat Mofaz into second place, but such surveys have been wrong in the past on party primaries.

* The winner needs 40 percent to avoid a two-person run-off a week later. Results are expected the day after voting.

* The new Kadima leader will not automatically become prime minister. Several steps must follow:

-- Olmert must resign, which he has pledged to do. Once he does, however, he remains caretaker premier until his successor, or someone else, has formed a new government -- a process that may happen almost instantly but could take months.

-- To form a government, the new Kadima leader will need a formal mandate to do so from President Shimon Peres. He can take up to 14 days to grant it.