BERLIN—The three main contenders to succeed Angela Merkel as chairwoman of Germany’s largest political party have called for a review of a planned gas pipeline between Germany and Russia, potentially putting the party at odds with the chancellor’s own government.
Since last Friday, the candidates have publicly questioned whether Germany should forge ahead with the controversial Nord Stream 2 project after Russian forces detained Ukrainian servicemen off the shores of the Russia-occupied peninsula of Crimea, sparking international condemnation.
Work is already well under way on the pipeline, which German authorities approved in March. When it comes online next year, it is supposed to channel gas from Russia directly to Germany under the Baltic Sea, doubling the capacity of an existing link.
If Ms. Merkel’s Christian Democratic Union takes a position against the pipeline project, it would break with more than a decade of bipartisan support for the project. Such opposition, while it may not immediately result in a change of policy, would make it harder for the chancellor to continue with a project that has sparked vehement opposition from the U.S. and a number of Germany’s European allies.
The CDU turning against Nord Stream 2 could also be the first test of the government’s cohesion after the party comes under new leadership. The center-right party’s junior partner in the coalition, the left-leaning Social Democratic Party, has been among the project’s strongest backers.
Friedrich Merz, Annegret Kramp-Karrenbauer and Jens Spahn—the main contenders in a vote for Ms. Merkel’s successor to be held at a party convention on Friday—have all said Germany might have to reconsider its stance over the pipeline in light of Moscow’s renewed international aggression.
Heiko Maas, the SPD foreign minister, rejected the suggestion this week, saying the pipeline couldn’t be abandoned and should instead be used as leverage in negotiations with Russia over the security of Ukraine. Former SPD Chancellor Gerhard Schröder, a champion of the project from the outset, is the chairman of Nord Stream’s shareholders’ committee.
Mr. Merz, a lawyer and former CDU parliamentary leader, and Ms. Kramp-Karrenbauer, an ally of Ms. Merkel, are considered front-runners. Ms. Kramp-Karrenbauer said in a televised debate that the political significance of the pipeline had to be reconsidered.
Mr. Merz went further, saying Russia’s repeated provocations in Ukraine were making it increasingly awkward for Germany to keep championing Nord Stream 2.
“The more this conflicts escalates, the more we have to discuss whether it’s really the right decision to build this pipeline,” Mr. Merz told a party gathering on Friday. “Putin must know that we will at least think about this issue if he continues to behave as he currently does.”
Mr. Spahn, the third candidate, told the Bild newspaper that Germany couldn’t unconditionally support the completion of the pipeline if the Kremlin didn’t alter its behavior.
Their comments have re-energized criticism of the project in Germany and abroad amid mounting discomfort at Russia’s military aggression in Ukraine, its hounding of the Kremlin’s critics abroad, its role in the Syrian civil war and its interference in U.S. and European politics.
Germany’s government spokesman in October accused Russia for the first time of directing a campaign of cyberattacks against Germany’s state institutions. A series of attacks beginning at least two years ago have targeted the parliament and key ministries.
After a cyberattack on the German defense ministry a month ago, U.S. officials approached German officials at various levels of government to warn about the dangers of making Germany more dependent on Russian gas imports through the Nord Stream 2 project, a person familiar with the events said.
“Many people inside Germany are coming to realize what most people outside of Germany already know: It is unwise to give Russia more influence over Europe’s energy security,” said Richard A. Grenell, the U.S. ambassador to Germany and a former member of the Trump campaign.
An official close to Ms. Merkel said the German government wouldn’t have any legal means of stopping the pipeline as authorities have already issued permits and construction was under way.
“We follow the political debate closely but we have all approvals and we stand on solid legal ground,” said Steffen Ebert, a spokesman for Nord Stream 2. “Germany is not a banana republic where politicians can stop commercial projects. We have law and order here.”
Experts say that the European Commission, the EU’s executive arm, has the legal means to stop the pipeline project. Manfred Weber, a member of Ms. Merkel’s conservative bloc who is likely to become the next president of the Commission, told the Journal that the project went against Europe’s interests and Berlin shouldn’t hide behind legal arguments.
“I believe that a project which will make Europe even more dependent on Russia’s gas is the wrong path to take. It must therefore be reconsidered,” he said.
—Andrea Thomas contributed to this article.
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