Isaac Herzog’s upset win Thursday over incumbent Labor chair Shelly Yachimovich for the party’s top spot has upended the political calculations of many.

With 15 members in Israel’s 120-seat Knesset, Herzog controls the third-largest faction in parliament, too small to lead the country but just large enough, at least on paper, to replace the entire right wing of Prime Minister Benjamin Netanyahu’s government should they threaten to topple it in order to forestall any impending peace agreement.

That simple mathematical fact — a government with Likud, Labor and the two liberal centrist parties Yesh Atid and Hatnua would have the slimmest possible majority, with 61 seats — has set the political system a-flurry with speculation over Herzog’s future steps. Will he seek to join Netanyahu’s coalition?

Even if he doesn’t take a seat in the cabinet alongside Netanyahu, the widespread assumption that such a move is now a possibility – his predecessor Yachimovich rejected the idea out of hand – means Netanyahu’s political maneuvering room has just expanded considerably, especially when it comes to the US-brokered peace talks with the Palestinians.

As they awoke to this new reality on Friday morning, many political actors who see their future tied up with Labor’s wasted no time in making their opinion known.

Israel’s perennial left-wing oppositionist, Meretz leader MK Zehava Galon, called on Herzog “to remain with Meretz in the opposition and not join Netanyahu’s coalition.”

Justice Minister and chief Israeli peace negotiator Tzipi Livni (Hatnua), who occupies, often uncomfortably, the left-wing edge of the coalition, urged the opposite.

After initially calling on Labor to leave “the stands” and join the coalition, later on Friday Livni issued a softer but nevertheless eager call for Labor’s direct involvement in advancing the peace talks.

Herzog’s election “has an added, special significance,” she wrote. “I believe Bougie [Herzog’s widely-used nickname] truly believes and is committed to a diplomatic solution, and I hope he will mobilize to help change our collective future here. The peace process demands real support, not conditional support,” Livni said in a jab at Yachimovich’s lukewarm statements that she would support Netanyahu’s peace efforts only if such a deal was imminent.

Livni concluded with an emphatic statement that bluntly suggested she’d prefer Herzog to several of her current coalition partners: “In short, Bougie, congratulations, and together we can stop the extremists.”

For his part, Herzog refused to satiate the speculation with any commitments.

“We just got elected. This isn’t the time” to discuss joining the coalition, Herzog’s spokeswoman Linda Sason told The Times of Israel Friday.

At his celebratory Friday morning press conference, when asked directly by an Israel Radio reporter if he would join the coalition, Herzog’s response suggested it was not off the table, though it would require a “clear, daring” act by Netanyahu: “I will serve as opposition leader. And I will meet with the prime minister when relevant… I have said before that if he makes a clear, daring step toward peace, I would be there. I stand by what I said.”

Herzog’s aloofness is understandable. One does not begin the delicate dance of coalition negotiations by appearing overeager for membership. And despite Livni’s plea that he not wait for a peace deal, but rather join the coalition to help construct it — the obstacle to that scenario is not Herzog, but Netanyahu. The prime minister has little to gain from abandoning his current right-wing coalition in favor of a narrower one with Labor as long as, in his view, the Palestinians remain implacably intransigent.

And finally, Herzog’s victory celebration is tempered with a grim sense of the deteriorated state of his new fiefdom. He leads a former ruling party that once reliably attracted over 40 Knesset seats, but has failed in two consecutive elections to get into the upper teens. He may be leader of the opposition, but only because the second-largest party, Yesh Atid, sits comfortably ensconced in the coalition.

He also feels the pressure of leading a party in which a huge majority of MKs sided with his opponent in the primaries, and which has a long history of undermining and quickly felling anyone who has the misfortune to stand at its head. Herzog’s defeat of Yachimovich marks the tenth ouster of a party leader in 21 years. No Labor leader has won reelection since Shimon Peres’s win in 1988.

Of course, it was this very sense of decline that led to Herzog’s dramatic victory with a 16 percent margin. His entire campaign was a vague appeal to the need to return to that past grandeur.

It is not at all clear that rushing to join the right-wing ruling government is the best way to return Labor to its once-storied role as leader of a popular, agenda-setting left.

The pressure is great, the stakes even greater. But those fateful decisions lie in the future. For now, Herzog can take comfort in knowing that the mere speculation about his future moves has already caused consternation on the right, where some MKs are sufficiently worried that they have already begun the campaign against Herzog’s entry into the coalition.

As the Likud’s Ofir Akunis said Friday morning in a Facebook comment dripping with sarcasm, and directed as much at Netanyahu as at Herzog: “Congratulations to the new chairman of the Labor party. I wish him many long years as leader of the opposition…”