Social Democratic Party, SPD, chairman Martin Schulz, first row left, sits with other party leaders on the podium during the party's convention in Berlinon Thursday. (Markus Schreiber/AP)

Germany's main center-left party on Thursday offered Chancellor Angela Merkel a way out of the country's months-long political stalemate while signaling that its price may be a radically different vision of Europe.

The Social Democrats (SPD) had repeatedly insisted they would not join Merkel’s conservatives in another grand coalition following an inconclusive September election. But on Thursday, delegates to a party conference in Berlin voted to allow talks to go ahead despite deep internal misgivings.

The reversal came weeks after Merkel's first attempt at forging a coalition collapsed, and a rerun of the alliance between Germany's two largest parties emerged as the only viable path to avoid another election.

German President Frank-Walter Steinmeier, a former SPD foreign minister, has been busy trying to bring the parties around on the idea of a renewed partnership ever since.

With Thursday’s vote, SPD delegates cleared the way for party leader Martin Schulz to begin negotiations with Merkel as soon as next week. Her party’s general secretary, Klaus Schüler, welcomed the decision as a step toward “a reliable and stable government.”

But the talks are not expected to yield agreement any time soon, with analysts suggesting that Germany may not have a new government until March. And the negotiations could still fail, given the array of issues on which the SPD and Merkel's conservatives disagree, including asylum policy, climate goals and pension reform.

Schulz on Thursday appeared to widen the gap between himself and Merkel in one critical area, Europe, by proposing a far more integrated club that mirrors another union across the Atlantic.

“I want a new constitutional treaty to establish the United States of Europe,” Schulz said.

The treaty, he said, would go into effect by 2025. European Union members that don’t accept the new agreement would be forced to leave the bloc, which currently stands at 28 members and is due to shrink to 27 with Britain’s scheduled exit in 2019.

The proposal by the former speaker of the European parliament represents a far more dramatic vision of European integration than any he had pitched during the German election campaign.

It is also sharply at odds with the cautious approach Merkel has taken toward reform at a time when skepticism about the European project is rampant across the continent and voters show little appetite for a vastly more powerful Brussels.

But Schulz’s push could add impetus to reform efforts being advanced by French President Emmanuel Macron. While he has not gone nearly as far as Schulz, Macron has detailed a far-reaching plan to more tightly bind users of the euro currency through a common finance minister and budget.

Macron had been expected to work with Germany on the proposal. But the uncharacteristic political uncertainty in Berlin has left the French president, at least temporarily, without a partner.

Schulz, who guided his party to its worst result of the postwar era in the September vote, on Thursday took responsibility for what he described as “a bitter defeat.”

In the hours after the election, Schulz had vowed that the SPD would go into opposition to more clearly distinguish itself from Merkel’s Christian Democratic Union.

But he was forced into an embarrassing about-face after Merkel's plans for a deal among her bloc, the pro-business Free Democrats and the environmentalists Greens fell apart.

Despite the fact that he leads a struggling and deeply divided party, Schulz was reelected as SPD leader on Thursday with 82 percent of the vote.

Luisa Beck contributed to this report.

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