BERLIN — Even in Germany, where consensus is highly valued, it had never happened before.

On Sunday, Martin Schulz was anointed leader of the Social Democrats, Europe’s oldest democratic party, with 100 percent of the valid votes cast at a special convention.

The result places Mr. Schulz, 61, a former president of the European Parliament, in pole position to unseat the world’s most powerful woman, Chancellor Angela Merkel, when the two face off on Sept. 24 in the national election, in which Ms. Merkel is seeking a fourth term.

Even before that, it could increase the tensions coursing through Germany’s relationship with President Trump’s administration. Already, Mr. Trump’s actions — and Britain’s decision to leave the European Union — have had an effect here, slowing the rise of right-wing populism as voters re-examine the value of the Continent’s unity.

But the rallying cries in Mr. Schulz’s 75-minute address to the convention seemed destined to irk an American administration that is already demanding more from its NATO allies. His best applause lines railed against buying more weapons, or argued that even Mr. Trump should hold fast to democratic values.