The Democratic Socialists of America claimed credit for Walz being picked in a thread on X.

Harris choosing Walz as a running mate has shown the world that DSA and our allies on the left are a force that cannot be ignored. Through collective action, DSA and the US left more broadly have made it clear that change is needed. DSA members organized in our workplaces and unions to realign the labor movement to support Palestinian liberation.

The Uncommitted movement, in which DSA members played crucial roles nationally and in multiple states, pressured the Democratic establishment into choosing a new candidate and backing down from a potential VP with direct ties to the IDF and who would have ferociously supported the ongoing genocide in Palestine. //

But what the DSA is telling us by this is that they seem to believe Walz was the more left-wing choice. That tells you everything you need to know about him, because they sure don't sound like they think he's a "centrist," as the media is trying to pitch him.