When Marx wrote that "A spectre is haunting Europe — the spectre of communism", was it? How influential was communism in 1848?

Put that quote into context:

Where is the party in opposition that has not been decried as communistic by its opponents in power? Where is the opposition that has not hurled back the branding reproach of communism, against the more advanced opposition parties, as well as against its reactionary adversaries?

Marx is saying that "Communist" was already popular as an insult and accusation to opposition movements in Europe, that governments would accuse their critics of being stalking-horses for communism.

In the very year that Marx wrote this, for example, the Second Republic of France in its brief radical phase, encouraged by Socialist politician Louis Blanc, established the National Workshops to provide guaranteed work for the unemployed. Once Blanc's influence had faded, the National Workshops, hated as a sign of government-provided make-work, were shut down in June of 1848, and General Cavaignac, fresh off of conquering Algeria, led troops to quell (i.e. massacre) angry workers in the streets.

You can see the point Marx was trying to make in his quote in this story. Louis Blanc, a socialist, advocated and briefly passed a radical reform, and it was swiftly undone in a mere four months because it was a bit too radical for most of the French political establishment.

Marx goes on, in the text, to say that if everyone is already accusing all of their foes of being "communists," that's a sign that communism is already a relevant political force and that it's time communists proclaim what their actual program is rather than having their program be defined by their enemies using the term as an insult.