Martin Schulz, leader of the German Social Democrats, waves after his speech at the SPD federal party congress on December 7, 2017 in Berlin | Carsten Koall/Getty Images SPD’s Martin Schulz wants United States of Europe by 2025 ‘Those who are against it will simply leave the EU,’ party chief tells conference.

BERLIN — Martin Schulz, the head of Germany's Social Democrats, wants to push for ever-closer European integration and turn the EU into a 'United States of Europe' by 2025.

“I want there to be a constitutional treaty to create a federal Europe," Schulz said Thursday during a speech at a party convention in Berlin, as he urged his party to clear the way for talks with Angela Merkel’s conservatives which could lead to a new German government and put an end to an unprecedented coalition deadlock.

The drafting process of such a constitutional treaty, Schulz said, should involve citizens across the Continent. Once drafted, it would "be presented to the member states, and those who are against it will simply leave the EU," he said, adding that Poland was already systematically undermining European values and Hungary was increasingly isolating itself.

Schulz' Social Democrats have become Chancellor Angela Merkel's last viable option to form a stable government after her talks with smaller parties collapsed. Later on Thursday, SPD delegates are set to vote on a motion submitted by the party's leadership laying out the prerequisites to enter negotiations with the chancellor.

I want a new constitutional treaty to establish the United States of Europe. A Europe that is no threat to its member states, but a beneficial addition. — Martin Schulz (@MartinSchulz) December 7, 2017

Schulz, a former head of the European Parliament, said a pre-condition for entering such talks would be "an about-face in European policy."

"The SPD is needed for a just and innovative Europe," he said. "Our country cannot afford another four years of European policy à la [former conservative Finance Minister] Wolfgang Schäuble."

Asked about Schulz’s speech at a press conference in Berlin, Merkel declined to respond to the specifics of his proposal but said the EU should focus on fixing its “weaknesses” and improving its “ability to act.”

“I think this ability to act should be in the foreground now, rather than defining a goal, however you choose to describe it,” she said.

“That’s why I’m concentrating on saying by 2025, we should have much stronger cooperation on defense, much stronger cooperation in the other issues I’ve mentioned,” Merkel said, citing topics such as research, foreign policy and education.

Other conservatives, however, were sharply critical of Schulz's vision. Alexander Dobrindt, a top lawmaker for the Christian Social Union, called the SPD leader a "Europe radical."

"Someone who wants to create the United States of Europe by 2025, who wants to dissolve nation states in the next seven years, and who wants to throw anyone who doesn't submit to this dictate out of the EU, must be called a Europe radical," Dobrindt said.

Emily Schultheis in Berlin contributed reporting.