FRANKFURT (Reuters) - Chancellor Angela Merkel’s preferred successor as head of Germany’s conservatives warned the Social Democrats (SPD) on Thursday that quitting the ruling coalition after a weekend regional vote would trigger a federal election.

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With the conservatives and SPD facing a second local election drubbing in two weeks, Sunday’s vote in the western state of Hesse could help decide how Merkel choreographs the end of her 13-year stewardship of Europe’s biggest economy.

SPD members are piling pressure on their own leaders to abandon the ‘grand coalition’ they reluctantly joined in March and reinvent themselves.

In unusually blunt language at a campaign event in the city of Frankfurt, the general-secretary of Merkel’s Christian Democrats (CDU), spelt out what this could mean.

“If this government were to break apart now, it would lead to new elections,” said Annegret Kramp-Karrenbauer, who Merkel promoted to the senior party post in February in a clear signal that she is the chancellor’s choice to succeed her.

Opinion polls suggest a snap election would probably hurt the conservatives and SPD most.

Merkel has been weakened by her 2015 decision to let more than a million migrants into Germany and -- while polls show her conservatives are still the biggest party -- their support has fallen to 26-27 percent, meaning a coalition with the SPD might no longer have sufficient support.

The environmentalist Greens, on 16-20 percent have overtaken the SPD in most surveys, and the far-right Alternative for Germany (AfD) remain strong with 15-17 percent.

In the Hesse election, both Merkel’s conservatives and the SPD look set to lose 10 percentage points compared to the last election in 2013, while the AfD is poised to enter the state legislature for the first time with 12 percent support, a new poll conducted for broadcaster ZDF showed.

SPD leader Andrea Nahles, in an interview with the RND newspaper chain, warned party members against overreacting to any losses in Hesse, and said it remained to be seen if Merkel’s conservatives and their Bavarian CSU allies could overcome the differences that have roiled the coalition in recent months.

Infighting within Merkel’s federal coalition, which nearly has hit the popularity of all three partners since they took office in March. The government came to the brink of collapse in July due to a row over migrant policy.

After coming second in an election in the southern state of Bavaria on Oct. 14, the Greens are also set to be kingmakers in Hesse, but big losses for the conservatives could spell the end of the conservative-Greens alliance in the state.

Merkel, in a final campaign speech in Hesse, urged supporters to go to the polls on Sunday and be “ambassadors” for continued conservative leadership in the state.

“Everyone who has the right to vote should do their part,” she said in the city of Fulda, which borders the former East Germany.

State premier Volker Bouffier told the rally it was critical to avoid a leftist alliance taking power.

“Either we keep this successful model that we have in Hesse or there is a leftist alliance,” he said. “Those are the alternatives. Nothing else matters.”