Thomas Oppermann, Bundestag faction leader of the German Social Democrats (SPD) | Sean Gallup/Getty Images German Social Democrats rule out NATO defense spending target Thomas Oppermann, the SPD’s parliamentary leader, doubles down on party’s opposition to defense spending target backed by NATO members.

The parliamentary leader of Germany's Social Democratic Party (SPD) rejected NATO's 2 percent of GDP defense spending target and called for strategic investment in the German armed forces in an interview published Thursday.

"We think this is the wrong way, and with the SPD in the government there will be no such thing," Thomas Oppermann told Rheinische Post.

Oppermann called for greater investment in Germany's Bundeswehr, rather than adhering to what he considers an arbitrary spending goal. "Defense expenditures will increase, but this must not be followed by an irrelevant quotas logic, but a comprehensive security logic," he said.

A new SPD position paper, which Oppermann is due to present in Berlin Thursday, will call for more parliamentary involvement in the development of the German armed forces, according to the Rheinische Post report.

Defense Minister Ursula von der Leyen on Monday defended the 2 percent spending commitment — which was agreed by NATO member states in 2006 — saying it was in Europe’s best interest that Berlin keeps its word on reaching the NATO target. The SPD’s rhetoric, she added, was part of a “totally messed up election campaign.”

In an op-ed co-authored with SPD leader and Merkel challenger Martin Schulz, the SPD official harshly criticized von der Leyen on Sunday for pledging to step up military spending to reach the NATO target by 2024.

That would require almost doubling the current defense spending level of €37 billion, making Germany the largest military force in Europe — a bad idea "because of our past," Schulz and Oppermann argued, suggesting moving forward on the development of a pan-European army force instead.

The SPD continues to trail the CDU by double-digits in the polls, ahead of the election on September 24.

This article has been updated.