Rosa writing to Henriette Roland Holst, August 2011 is firm, chastising and correct, and it seems effective, Holst forms a new group which later enters the SPD, although I know zilch about their influence after joining:
“Your long silence was all the more painful for me because I had to assume for various reasons you were dissatisfied with the general situation and consequently with your own. Now I hear: things are going well with you personally and with your health, but you want to leave the SDAP. The first things make me truly happy, but the last one—no! You certainly know I was strongly opposed to your staying in the party at the time when the others left it. I was and am of the opinion that you should all stay together—inside or outside. Fragmentation of the Marxists (not to be confused with having differences of opinion) is fatal. But now, when you want to leave the party, I would like with all my might to prevent you from doing that. You do not want to join the SPD, or so I hear. I’m not able to judge whether that is correct or not. If you want to join the SDP but can’t, enough said. But then by leaving the SDAP you are leaving the Social Democratic movement! You can’t do that, none of us can! can’t be outside the organizations of the musses, out of contact with them. The worst working-class party is better than none. And times can change. In few years a stormy period could sweep away the opportunist muck in Holland or even in all of Europe. But a person can not wait for such times from the outside, one must carry on the fight within, no matter how sterile or fruitless the effort may seem—to the very end. If you stay outside, You you are finished, dead for the political movement. Don’t do that! You also have responsibilities toward the International. Stay with the rank and file, that is our duty, we are all soldiers. I warn you against taking a false step”.
[Fn… Holst left the SDAP in 1911, founded the Revolutionair Socialistisch Verband (Revolutionary Socialist Association). and together with that group joined the SDP in 1916.]