Suddenly a Socialist as the next European Commission president is not such a crazy idea.

Just a few weeks ago, the notion of the EU's top job being filled by a candidate from the center-left Party of European Socialists seemed preposterous. No Socialist has led the EU's executive arm since 1999, when Manuel Marín of Spain filled in for six months as a replacement for Jacques Santer, the Luxembourgish conservative who resigned in a corruption scandal. And no Socialist has served a full term as Commission president since Frenchman Jacques Delors, who held the job from 1985 to 1995.

But the U.K.'s failure to quit the EU as expected, allowing British Labour candidates to run in the European Parliament election, has boosted the number of seats that the Socialist group is likely to win, according to POLITICO projections, assuring that it will finish no worse than second to the center-right European People's Party (EPP).

Some Socialists, buoyed by recent electoral successes in Spain, Finland and Sweden, even hold out hope of claiming the top spot. And the rise of a new progressive political force led by French President Emmanuel Macron and Dutch Prime Minister Mark Rutte is putting the long-dominant EPP, and its nominee, Germany's Manfred Weber, on the defensive.

"With Socialists, Liberals, Macronists and Greens, we can be more than the EPP," said Marco Aguiriano, the secretary of state for the European Union in Spain's Socialist government.

"It's time to broaden pro-European alliances," — Marco Aguiriano, Spain's secretary of state for the European Union

At the final major debate of lead candidates, or Spitzenkandidaten, the Socialist nominee, Commission First Vice President Frans Timmermans, made an open pitch for a left-wing alliance, including the Liberals, Greens and the far left, and stretching from Macron to Greek Prime Minister Alexis Tsipras. Such an alliance would aim to break the monopoly of the EPP, which now holds all three of the EU's top jobs: the presidencies of the Parliament, the European Council and the Commission.

"It's time to broaden pro-European alliances," Aguiriano said. "It’s time to better distribute jobs."

But the Socialists' best bet for clinching the top job may be a different coalition than the one Timmermans has publicly floated.

Red lines

Party officials, and other EU insiders, said that a Macron-to-Tsipras alliance would be difficult if not impossible to achieve, given various red lines. For example, an official with Macron's La République En Marche party said it could never work with the French far-left party, France Unbowed. That would seem to preclude an alliance between the new liberal-centrist group that Macron is working to form in the European Parliament, and the European United Left/Nordic Green Left family.

Philippe Lamberts, a Belgian MEP who is co-leader of the Greens group, cautioned that neither the Socialists nor anyone else could take his party's support for granted.

“What coalition? The Socialists are not going to be more hegemonic than before. What does progressive even mean? It means that mainstream forces say: ‘We want the maximum power for us,’” Lamberts told POLITICO in an interview. "We, as Greens, will be no one’s instrument, neither the Socialists nor Macron. We are an autonomous force. We are neither on sale nor for rent.”

Instead, a Socialist Commission chief may be most likely to emerge from a more mainstream, pro-EU majority long expected as the inevitable outcome of this European Parliament election, in which far-right, nationalist forces are predicted to pick up a substantial number of seats. The more mainstream majority would involve the EPP, the Socialists, the new liberal-centrist group and the Greens.

Despite his fighting talk, Lamberts also acknowledged that "all parties will need to get around the table and agree on something."

If the Socialists, Liberals and Greens can find common ground on a candidate for Commission president, the EPP would face pressure to go along with them — even if the center-right group is the largest in a fragmented Parliament.

Already, Europe's two most prominent Socialist leaders — Prime Minister Pedro Sánchez of Spain and Prime Minister António Costa of Portugal — have signaled a willingness to work with Macron.

Sánchez and Costa met with Dutch premier Mark Rutte and his Belgian counterpart Charles Michel, two liberals and allies of Macron, on the sidelines of an EU leaders' summit in Sibiu, Romania earlier this month. Costa also sent a video message in support of a centrist political rally that En Marche organized in Strasbourg earlier this month.

"Costa sent a video message to us. That shows we are compatible with these people, and with the Greens too," the En Marche official said. "We have them on our list. We have clear ideological convergences with these people."

The official added, "Our primary objective is no Weber. He is a bad political symbol for Europe in this moment."

Whither Timmermans?

Whether such a common candidate of Socialists, centrists, liberals and Greens would be Timmermans is a separate question. It is up to the 28 heads of state and government on the European Council to agree on a nominee, who must then be confirmed by a majority of Parliament — 376 out of 751 votes.

The national leaders on the Council have warned for more than a year that legally they cannot — and will not — be bound to the so-called Spitzenkandidat or "lead candidate" system. In other words, a Socialist may now have a chance, but it may not end up being Timmermans. A non-EPP nominee could also come from the new centrist-liberal group.

Among Socialists, former Danish Prime Minister Helle Thorning-Schmidt is frequently mentioned as a possible candidate for a top EU job. Portugal's Costa has also been described as a possible surprise contender — though he has insisted that he will not take a job in Brussels. "It is very complimentary, but I am not a candidate for anything other than the functions I play in Portugal," Costa said at a recent EU summit in Sibiu, Romania.

"There are so many different issues at stake ... it's such a complex tableau of things, which will involve a lot of players," — Janis A. Emmanouilidis, director of studies at the European Policy Centre

Also, Spanish Foreign Minister Josep Borrell, a former president of the European Parliament, is the No. 1 candidate on the list of the Spanish Socialist Workers' Party, and widely expected to get a senior post in Brussels, either in the Commission or the Parliament.

Other potential candidates whose names surface frequently in discussions among EU insiders include two liberals who are still in office: Rutte and Danish Prime Minister Lars Løkke Rasmussen. Belgium's Michel is also sometimes mentioned. In addition, European Competition Commissioner Margrethe Vestager is on the official slate of the ALDE alliance of liberal parties for top jobs.

Echoing many other party operatives, the En Marche official said much would hinge on the European Parliament election. Even if EU leaders have insisted they will not be bound by the lead candidate process, they know they must find a nominee who can win a majority in Parliament.

Before the outcome of the election is known, the official said, it is impossible to predict who might win the Commission presidency or form a coalition.

"It’s too soon to say, it will all depend on results," the official said.

Political operatives hoping to break the EPP's lock on the EU leadership said their success would depend on numerous factors. If Hungarian Prime Minister Viktor Orbán's Fidesz party, already suspended by the EPP, were to leave the group entirely, it would cut the EPP's numbers by more than a dozen seats. French Socialists are currently polling below the 5 percent threshold needed to win seats in the Parliament — a remarkable decline — but if they can scrape above that barrier, it would boost the left's numbers.

Janis A. Emmanouilidis, the director of studies at the European Policy Centre, a nonpartisan think tank in Brussels, said that such a highly complex array of factors will be at play in the negotiations over the EU's future leadership roster that it is virtually impossible to make any predictions at this stage.

"There are so many different issues at stake — personnel issues, policy issues, institutional issues, it's such a complex tableau of things, which will involve a lot of players," Emmanouilidis said in an interview. He said he expects the Spitzenkandidat process would be a consideration but would not ultimately determine the outcome.

"It will be a puzzle, which has to be put together and it will happen step by step," Emmanouilidis said. "This thing will be moving very, very slowly."