President Obama will begin his one-year effort to achieve Middle East peace on Wednesday, joining a long list of his predecessors who have tried to achieve a comprehensive peace between Israelis and Palestinians.

But unlike the presidents before him, Mr. Obama will know within three weeks whether the two sides are serious this time about reaching a deal.

This latest peace effort — the ninth since 1979, when negotiators identified the final status issues that would bedevil them for three decades — is to begin on Wednesday night, when Mr. Obama sits down to dinner with Prime Minister Benjamin Netanyahu of Israel; the Palestinian president, Mahmoud Abbas; President Hosni Mubarak of Egypt; and King Abdullah of Jordan at the White House.

Mr. Obama, administration officials said, will call on the four leaders to do all they can to settle, within a year, the final status issues: the fate of Jerusalem, the borders of a Palestinian state, the right of return for Palestinian refugees who fled their homes and the issue of Israeli security.

But on Sept. 26, Israel’s 10-month moratorium on settlement construction will expire. Mr. Netanyahu appears unlikely to extend it, Israeli and American officials said. And Mr. Abbas has said that he will withdraw from negotiations if settlement activity resumes.

“This becomes the first test of the intentions of the two sides, a test of whether they’re serious,” said Martin S. Indyk, the former American ambassador to Israel and Middle East peace negotiator.

If Mr. Netanyahu and Mr. Abbas stick to hard-line positions, the peace process will be dead just after this attempt is born, Mr. Indyk and other Middle East experts said. But if they are able to come up with a way around the Sept. 26 moratorium hurdle, “they will have created a more positive environment” for tackling the big issues, Mr. Indyk said.

Or put more simply, they will have shown the world that this time, they mean business.

Aware of the looming timetable, Mr. Obama has sent Dennis Ross, the National Security Council’s top Middle East official, to Israel to try to figure out a way to negotiate on the issue of the moratorium expiration, American and Israeli officials said.

Those officials, who spoke on the condition of anonymity because of the delicacy of the talks, said that discussions were under way on a number of possible solutions. They include trying to get a promise from Mr. Netanyahu that Israel will exercise restraint in settlement construction, perhaps allowing construction only within existing West Bank settlement blocks, but no housing starts beyond those blocks.

Such a plan could also include early “confidence building” concessions from Israel on a few additional issues of concern to the Palestinians, officials said, including agreeing to limit Israeli Army incursions into Palestinian-controlled areas in the West Bank, and transferring key areas in the West Bank to Palestinian control before a final agreement is reached.

“America’s biggest concern right now is to preserve the talks between now and Sept. 26,” said Robert Malley, the Middle East and North Africa program director for the International Crisis Group.

But getting these issues resolved early on could put Mr. Obama in a better position than his predecessors for reaching an eventual peace deal, Middle East experts said, because he would have cleared a big obstacle, and because each side would have more confidence in the other’s willingness to reach a deal.

If the two sides get beyond Sept. 26, tough issues will remain. While some Middle East experts say the contours of a peace deal are well known — the two sides have, after all, been grappling with it since 1979 — the question of Israeli security once a Palestinian state is established could also derail an agreement.

“Before we address borders, we have to address security,” Michael B. Oren, Israel’s ambassador to the United States, said in an interview. “The outcome of the final status issues is in many ways contingent on resolving Israeli security issues.”

For instance, Mr. Netanyahu has made clear that he would want to place Israeli soldiers on the borders with Jordan and Egypt to stop any potential arms traffic from getting into a future Palestinian state.

Israeli officials do not like the idea of placing an international force along the borders that does not include Israelis; they have balked at suggestions of a NATO force, for instance.

Israeli officials also say that they do not want Palestinians to have a national army. Unsurprisingly, Palestinian officials have balked at both of those restrictions, although some Middle East experts say that Palestinian negotiators might be willing to allow Israeli forces to operate within a future state if there was an understanding that Israeli defense operations would be phased out over time.

“So that compromise that Bibi might accept is an international force and an Israeli presence that would only be phased out as conditions permit, after a proven record of nonsmuggling of militants or arms,” said Mr. Malley, using Mr. Netanyahu’s nickname.

As for the brewing quarrel over a Palestinian army, Mr. Malley said that the Palestinians had indicated that they would be willing to accept a state with “limited arms,” although exactly how that would be settled remains to be seen.

Iran’s nuclear ambitions could also derail the talks. While the Obama administration has assured the Israelis that Iran is at least a year away from the “breakout capability” that would allow it to pursue a nuclear weapon, Israeli officials maintain that Iran’s pursuit of a nuclear weapon and the peace process are inextricably linked.

“In practical terms, if Iran gets the bomb it will deal a monumental blow to the peace process,” Mr. Oren said.