Israel's defense minister said Wednesday that he would use his political clout to bring down the coalition government if Prime Minister Ehud Olmert does not step aside to face possible bribery charges.

The ultimatum by Ehud Barak, a former prime minister who heads the coalition's second-biggest party, raised pressure on the Israeli leader to resign or at least temporarily step aside before a criminal investigation runs its course.

Israel is abuzz with speculation about how Olmert might fall, who would replace him and with what effect on peace talks with Syria and the Palestinians. Here are some questions Israelis are pondering: How did Olmert get into this jam?

Israeli leaders have no immunity from investigation or indictment. Amid growing popular disgust with official corruption, Israel's independent state controller had opened several inquiries into Olmert's conduct in office before his rise to the prime minister's job in 2006. Evidence that Olmert took cash-stuffed envelopes from wealthy New York contributor Morris Talansky came to light in one of those investigations. Talansky's recollection in court Tuesday of payments totaling about $150,000 over 15 years was the most politically damaging testimony yet. What is Olmert's defense?

He says he took the money for legitimate campaign expenses, not political favors, and will prove his innocence. Polls show most Israelis disbelieve him. By promising this month to resign if indicted in the slow-moving inquiry, Olmert had hoped to buy several months' time and keep a decision about his fate in the hands of Israel's independent attorney general, not those of rival politicians. Why is his coalition partner moving against him now?

Barak wants to be prime minister again and, in the wake of Talansky's testimony, cannot afford to stick up for Olmert. Potential rivals for the post were already calling for Olmert's resignation. In distancing himself from his boss, Barak said Olmert cannot lead peace negotiations and look after Israel's security needs while grappling with a criminal inquiry. He called on Olmert to resign or take a leave of absence, but he set no deadline.

What could Barak do to bring him down?

Barak's leftist Labor Party has 19 of the 64 parliamentary seats held by Olmert's coalition. By threatening to pull out, Barak is trying to put the onus on Olmert's centrist Kadima party to take the lead in ousting him. Labor's withdrawal would reduce Olmert's supporters to a minority in the 120-seat parliament, increasing the likelihood that the government would fall long before his four-year term ends in 2010. How could that happen?

With 61 or more votes, parliament could dissolve itself and call new elections within five months. Or it could pass a no-confidence motion and ask President Shimon Peres to assign one of its members to try to form a new government that could command the support of a majority of the current parliament. In either case, Olmert could continue running the country as a caretaker until a new government is formed.

What are Olmert's options?

He could cling to his job. That's what his aides, in answer to Barak's ultimatum, said he will do. They remember that Barak has threatened before to leave Olmert's coalition but failed to follow through.

Olmert could declare himself "temporarily incapacitated" and turn the office over to his second in command, Foreign Minister Tzipi Livni. Should Olmert fail to resume his duties after 100 days, parliament would be required to call new elections.

Or he could resign and call new elections. People in his party say Olmert might choose that route rather than tapping Livni, a Kadima rival who has irritated him by challenging his leadership. Who is maneuvering for his job?

Besides Livni and Barak, there's former Prime Minister Benjamin Netanyahu, who heads the right-wing opposition Likud Party. Polls make Netanyahu the current favorite in an election and show a slip in Barak's popularity since he became defense minister last year.

Livni's reputation for honesty gives her an advantage over Barak and Netanyahu, who have been investigated for alleged campaign financing violations but never charged. Livni's more seasoned rivals in Kadima, including Cabinet ministers Meir Sheetrit, Shaul Mofaz and Avi Dichter, also covet Olmert's job. How does all this affect the peace talks?

Palestinian negotiators worry that early elections in Israel would disrupt the talks for months and undermine public support for Mahmoud Abbas, the U.S.-backed Palestinian Authority president who is staking his leadership on a peace accord with Israel by the end of this year. If Olmert hands over the government to Livni, his chief negotiator with the Palestinians, the talks begun in November might have a better chance of bearing fruit.

Israel's recently disclosed negotiations with Syria through Turkish mediators are being conducted by Olmert's office and depend more directly on his tenure. But Barak and Livni support the talks and might continue them as prime minister.

Netanyahu is a fierce critic of reported Israeli concessions in talks with the Palestinians and Syria. His election would mean a harder Israeli line in both sets of negotiations, making progress more difficult.