This finally happened after the SPD voted for war credits in 1914, those who opposed split and formed the sparticists. The SPD was once a party that collaborated with Karl Marx on frequent occasions. As a side note, one of the key members- wilhelm liebknecht (not to be confused with his son Karl) and Karl Marx once went on a drunken rampage through London…
Anyways, here you can see Rosa Luxemburg and August Bebel, an indication of the revolutionary current of the party:
Friedrich Ebert became leader of the SPD following the death of August Bebel (standing to the left of Luxemburg) in 1913. Ebert supported the German war effort and the imprisonment of anti-war socialists including his former teacher, Luxemburg. Ebert became German Chancellor on 9 November 1918 during the German Revolution and was voted first President of the German Republic by the National Assembly at Weimar on 11 February 1919, a post which he held until his death on 28 February 1925. As Chancellor, Ebert was responsible for the crushing of the ‘Spartacist Rising’ in January 1919, which culminated in the murders of Rosa Luxemburg and Karl Liebknecht. Although he did not publicly endorse the murders, Ebert was considered responsible by the Communists and other left-wingers.
We should take note of how revolutionary parties can turn opportunist and reformist. The betrayal of the Ebert and the SPD can never be forgotten…
Death to reformism!
ACA The Underground 
How could a communist party, even allow someone like that in their party? And, how could actual communists all just let him do what he advocated for?
Whom are you referring to?
Ebert
Well, he had the support of the party a he was vice chairman. I’m not to sure on the specifics. Karl Liebknecht was the only one who opposed war credits in the Reichstag, and he left to form the sparticists.
So there was a way for non-communists to enter the party, and if that is avoided, we can avoid something like the SPD degeneration?
Then people could just pretend to be communist. The best way to prevent the degeneration of the party is to keep to a centrist line, and make it so the power is ‘spread out’.
What does a centrist line mean? Like a standard belief?
Apologies- a leftist line.
Think of Luxemburg’s position in the SPD.
A better example would be the post Lenin Bolshevik party. Trotsky’s left opposition and Bukharin’s right opposition, and the centrist faction of Stalin.
So expel those who are not a part of the leftist line?
Possibly. Or give them a voice, but not a vote.