BERLIN, GERMANY - FEBRUARY 07: German Chancellor Angela Merkel in the course of a special faction meeting, on February 07, 2018 in Berlin, Germany.

Florian Gaertner

The German government's smaller coalition partner, the Social Democratic Party (SPD), has said it could quit the alliance if Chancellor Angela Merkel is forced out of office by the new leader of her party, the Christian Democratic Union (CDU).

The politician seen as Merkel's successor as leader, Annegret Kramp-Karrenbauer, announced on Monday that she would step down from the CDU chair, creating a leadership vacuum at the top of the party and setting in motion a new leadership contest that's set to take place by the summer.

Merkel was expected to remain chancellor of Germany until her term in office ends in 2021 but if one of her political rivals is elected to the CDU leadership, it could be difficult for her to see out the full term.

The secretary general of the SPD, which is in a coalition with the CDU and its Bavarian sister party, the Christian Social Union (CSU), said Wednesday that if Merkel is forced from the chancellorship, the party could pull out of the so-called "Grand Coalition." That could trigger a full general election, ahead of a vote scheduled to take place before October 2021.

"Angela Merkel is the incumbent chancellor. We went with her into this coalition. And with her we will also leave this coalition regularly for the next election date," SPD General Secretary Lars Klingbeil said, according to Der Spiegel.

"We want to continue working together with the (Grand) Union in the federal government. This government is elected until autumn 2021," he said.

Klingbeil's comments come amid heightened uncertainty in the CDU over its future leader and whether he or she (although all the favorites for the job are men) will continue Merkel's more centrist approach to government or will take a more traditional, conservative position to try to counter a loss of voters to the right-wing Alternative for Germany party.

One of the favorites for the CDU leadership is Friedrich Merz, who is seen as a rival of Merkel. He is supported by party members that want a change from Merkel's centrism and were unhappy at some of her more liberal policies, such as her open-door policy during Europe's migrant crisis in 2015.

Armin Laschet, the premier of the state of North Rhine-Westphalia, and Health Minister Jens Spahn are also main contenders for the role and are both seen as continuing Merkel's centrist stance.

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If Merkel is forced out by her successor then German government will fall, coalition party warns - CNBC