It's actually two paragraphs.
Purging the Left of communists became a longstanding practice, having injurious effects on various progressive causes. For instance, in 1949 some twelve unions were ousted from the CIO because they had Reds in their leadership. The purge reduced CIO membership by some 1.7 million and seriously weakened its recruitment drives and political clout. In the late 10940s, to avoid being "smeared" as Reds, Americans for Democratic Action (ADA), a supposedly progressive group, became one of the most vocally anticommunist organizations.
The strategy did not work. ADA and others on the Left were still attacked for being communist or soft on communism by those on the Right. Then and now, many on the Left have failed to realize that those who fight for social change on behalf of the less-privilege elements of society will be Red-baited by conservative elites whether they are communists or not. For ruling interests, it makes little difference whether their wealth and power is challenged by "communist subversives" or "loyal American liberals." All are lumped together as more or less equally abhorrent.
An important thought here is in understanding what many communists always called engaging in "struggle".
When Lenin says that communists should engage in "struggle" against x thing he is explicitly stating that communists should oppose that thing to the highest degree possible, and that means doing so to the most extreme of degrees.
The right understands this and it engages in "struggle" effectively. It is engaged in struggle against the left, the whole left, anything that is left of itself. It will do so to the most extreme of degree, if something benefits them, they will do it, as it is helping in their struggle.
Liberals do not engage in struggle. They make the error in assuming the right act in good faith, that they will not use any and everything they can to their advantage, that they will not call the liberals communists when they clearly are not. They ignorantly refuse to engage in rightwards struggle.
Ironically, they WILL engage in leftwards struggle. They know which side their bread is buttered on.
The left must engage in struggle. And here is a critical part -- the left must engage in struggle against EVERYTHING to its right, libs and socdems included (but I repeat myself).