TEL AVIV — Ehud Olmert, the former prime minister who has spent the last several years battling corruption charges, is plotting a comeback that analysts say offers the best hope of uniting Israel’s fragmented political center, but also shows the opposition’s desperation in trying to block the seemingly inevitable re-election of Prime Minister Benjamin Netanyahu.

A tanned, fit Mr. Olmert has spent the week since Mr. Netanyahu called elections for January, rather than next October, holed up in his unmarked office in an upscale high-rise here, smoking cigars as a parade of politicians and pollsters, money men and lawyers, makes the pilgrimage to take his pulse or talk strategy. A Jerusalem Post poll published Friday suggested that if he could persuade the leaders of several centrist factions to join his ticket, it would beat Mr. Netanyahu’s Likud Party by several seats.

That is a very big if. There is also an intense legal debate over whether Mr. Olmert should even be allowed to run, given his conviction this summer for breach of trust and a separate trial under way in which he is accused of taking bribes in a real estate deal. Pundits have started to point out that before he left office under the corruption cloud in 2009, his popularity had plummeted to the single digits.

But it is clear that Mr. Olmert, 67, wants badly to run on a platform accusing Mr. Netanyahu of spoiling Israel’s relations with the United States, scuttling the prospects of peace with the Palestinians by expanding Jewish settlements and taking the wrong tack on Iran. Analysts say he is the only candidate seen as a credible alternative to Mr. Netanyahu and that his candidacy — if he manages to unify the cadre of centrist leaders he has been courting — would create the kind of compelling story that could shake up what otherwise looks like a moribund campaign landscape.

“Olmert’s the one person who, with all of his faults, has the experience and has the knowledge,” said Gadi Wolfsfeld, a professor of political communication at the Interdisciplinary Center in Herzliya. “It would be a major challenge to Netanyahu, and the only realistic challenge to Netanyahu.

“New candidates never have much credibility here,” Mr. Wolfsfeld added. “It’s a very green kind of political system: we tend to recycle the same leaders over and over.”

Monday night, Israel’s Parliament was scheduled to dissolve itself and officially set the balloting for Jan. 22, creating a lightning campaign calendar that many here see as Mr. Netanyahu’s effort to derail Mr. Olmert from gathering momentum.

Besides the question of who will lead the center, there is a separate drama unfolding over whether Aryeh Deri, who is also returning to politics after a corruption conviction, will once again lead the ultra-Orthodox Shas Party or form a separate movement. The surprise announcement on Sunday that Moshe Kahlon, a beloved minister in Mr. Netanyahu’s party, would not run for re-election further tilts that faction to the right.

It remains unclear whether the tiny independence party led by Ehud Barak, the defense minister and former prime minister who recently suffered a rift with Mr. Netanyahu, can win enough seats to return him to the government.

A close adviser to Mr. Olmert, speaking on the condition of anonymity, said that Mr. Olmert would make a decision by the end of next week. In the coming days he plans to meet with Tzipi Livni, his former foreign minister who lost her leadership of their Kadima Party this spring, and Gabi Ashkenazi, the respected former chief of the Israeli military, a candidate for defense minister. Along with them and the current leader of Kadima, Shaul Mofaz, Mr. Olmert is recruiting outspoken critics of Mr. Netanyahu’s aggressive posture toward Iran, including Meir Dagan and Yuval Diskin, the former heads of Israel’s intelligence agencies.

One who vowed not to join Mr. Olmert is Yair Lapid, a journalist who has spent a year building a personality-driven, outsider’s campaign. But Mr. Olmert, who was close friends with Mr. Lapid’s father, is confident they could form a coalition after the election.

Avigdor Lieberman, the current foreign minister and head of the Yisrael Beiteinu Party, is also seen as ripe for coalition-building, because Mr. Netanyahu’s exit from politics would create space for Mr. Lieberman to lead the right wing.

“He says to anyone who wants to listen, if there was anyone else in Israel, a politician or a leader who would have a realistic chance to replace Netanyahu, he would stay out of the system and support him with all his heart and abilities,” the adviser said of Mr. Olmert. “The question is, is Ehud himself able to make it? If you unite all the forces, and you run together, as a group, there is a good chance.”

First, though, there is the legal hurdle. Though he was acquitted of two more serious charges, Mr. Olmert was fined and given a one-year suspended jail term for breach of trust last month. Most legal experts agree that this allows him to run for Parliament, but not become a cabinet minister. They differ, though, on whether the prime minister is, technically, a minister, or a Parliament member empowered by the president to form a government.

Plus, what if he were elected and then convicted in the real estate trial?

“Legally speaking, he can run,” said Suzie Navot, a professor at the Comas School of Law in Rishon LeZion. “I think people who have been convicted in criminal cases that deal with the breach of trust with the public — he was not convicted in a road accident — we should not accept them back. But this is an ethical problem, not a legal one.”

Then there are the politics. While the Jerusalem Post poll published Friday indicated that Mr. Olmert could win 31 Parliament seats compared with Mr. Netanyahu’s 27, the key question in Israel is who can form a 61-member coalition afterward. (Ms. Livni’s Kadima won more seats than Likud in 2009, but Mr. Netanyahu outflanked her in making deals with rival factions.) The same poll showed all center-left parties totaling 54 seats, leaving the balance of power in the hands of Shas, Yisrael Beiteinu and other parties now aligned with Mr. Netanyahu.

“It’s a very simple calculation, very simple mathematics,” said Tamir Sheafer, a political science professor at Hebrew University. “Whether all the parties from the left will go together or run alone it doesn’t matter much, it just reshuffles the combination of the left, but they do not take enough voters from the right.”

Some have begun to worry that Mr. Olmert’s deliberations are distracting from less complicated, more realistic candidacies. The left-leaning Haaretz newspaper, no friend of Mr. Netanyahu, published an editorial on Sunday titled “No to Olmert,” saying, “Israel is not that corrupt and desperate.”

In his Monday column, Shalom Yerushalmi, a political analyst for the Maariv newspaper, said Mr. Olmert ultimately would not run “unless virulent political bacteria and insane ambition overcome simple rational consideration.”

Amnon Abramovich, a political commentator for Israel’s Channel 2 News, said he does not think Mr. Olmert has a chance. “He can make the campaign more sexy and more vibrant and more vital,” he said in an interview, “but I don’t think he can win.”