A warning (for comments I guess). I really like to answer rhetorical questions. The answers are quite long. I can semi remember one. Being clever (as he often is*) Matthew Yglesias asked how and when the Democratic Socialists became bitter adversaries of the Social Democrats, which is odd since both phrases are translations of the same polysyllabic German word. I happened to know the approximate answer (I didn’t recall the exact date which turns out to be a few years later than I thought).
As is usual, the Socialist Party split over a war — in this case the US-Vietnamese war. The Social Democrats supported US involvement in the war. The Democratic Socialists opposed it. This is a common reason for Socialist parties to split. It turns out that this occurred very late — in 1972. I would tend to guess that views of George McGovern were related.
The split is, of course, quite bitter. So, for example, this Wikipedia article, has the warnings
|The political neutrality of this article is disputed. Relevant discussion may be found on the talk page. Please do not remove this message until conditions to do so are met. (November 2021)
|This article may be unbalanced towards certain viewpoints. Please improve the article by adding information on neglected viewpoints, or discuss the issue on the talk page. (November 2021)
Anyway, it notes the basic facts
Social Democrats, USA (SDUSA) is a social-democratic organization established in 1972 as the successor of the Socialist Party of America (SPA). The SPA had stopped running independent presidential candidates and consequently the term “party” in its name had confused the public. Moreover, replacing the “socialist” label with “social democrats” was meant to disassociate the group from the Soviet Union.
SDUSA, which was fiercely anti-communist, pursued a strategy of political realignment intended to organize labor unions, civil rights organizations and other constituencies into a coalition that would transform the Democratic Party into a social-democratic party. The realignment strategy emphasized working with unions and especially the AFL–CIO, putting an emphasis on economic issues that would unite working class voters. SDUSA opposed the Senator George McGovern‘s “New Leftist” approach, pointing to the rout suffered in the 1972 presidential election. As a result, some SDUSA members, like Penn Kemble and Joshua Muravchik, were associated with neoconservatism. SDUSA’s activities have included sponsoring discussions and issuing position papers. SDUSA has included civil rights activists and leaders of labor unions such as Bayard Rustin, Norman Hill and Tom Kahn of the AFL–CIO as well as Sandra Feldman and Rachelle Horowitz of the American Federation of Teachers (AFT).*
Internationally, the group supported the dissident Polish labor organization Solidarity and several anti-communist political movements in global hot spots.
SDUSA’s politics were criticized by former SPA chairman Michael Harrington, who in 1972 announced that he favored an immediate pull-out of American forces from Vietnam and coined the term “neoconservative”. After losing all votes at the 1972 convention that changed the SPA to SDUSA, Harrington resigned in 1973 to form the Democratic Socialist Organizing Committee, the forerunner of the Democratic Socialists of America.”
So it happened in 1973 (I would have guessed about 5 years earlier).
*non Irony alert
**One might also hetorically ask why the American Federation of Teachers and the National Education Association can’t stand each other.