Though it runs a bit deeper than that, in my view.
The 'further left' traditionally take as their chief enemy parties and political figures of the center left, rather than parties and figures of the right. This is because a strong center-left party balks any possibility of revolution. It will have the allegiance of most working people, because it will bring them real benefits in their lives, and by doing so, will shut off influence of the 'further left' by making it clear measures well short of the desperate expedients the 'further left' prescribes for improving the lot of working people are not necessary.
Because center-left parties do uphold the present order of society, the 'further left' sees them as obstacles to its desires quite as much as any reactionary party on the right. Thus you have the 'not a dime's worth of difference' line that views our two major parties as interchangeable. Since the 'further left' cannot comprehend how working people could possibly form an honest attachment to rightist parties, their view comes to be that center-left parties are their chief obstacle to mass support from working people, and they imagine that if center-left parties are broken, they will inherit the mass support of working people, and thus become predominant. Then it will be the time to deal with the reactionary right, but until it is the 'further left' which has undisputed leadership of working people, the reactionary right cannot be dealt with properly.
Properly, here, indicating a policy guided by the slogan quite popular in the radical salad days of the seventies: 'What's the solution? Revolution!' Few nowadays on the 'further left' dream of an actual, barricades and snipers and car-bombs sort of armed revolution, but they do envision a complete overthrow of existing economic and social arrangements. One of the things they fail to understand about working people, and people on the lower rungs of the economic scale generally, is that people who have not much but do have a little are extremely reluctant to put the little they have at risk, and they know that in turmoil and tumult that little will be at risk. There are strains of the 'further left' which do have some understanding of this, and their view is that working people must be made to lose that little they have now, and lose it to the unmitigated predation of the reactionary right. Only then, when they have nothing to lose, will working people be ready for revolution under the banner of the 'further left'. This provides such people still another reason to oppose and demolish center left parties, as these do mitigate the suffering the right would impose on working people, and so are the chief force in balking revolution. These elements view an initial triumph of the reactionary right as an essential step in their own program to achieve revolution, and so are actually quite pleased by the reactionary right achieving political success at the expense of center-left parties.
"From Bernies perspective, dropping out of a race once you have no chance of winning is peculiar behavior that can only be explained by the work of a hidden hand. For most politicians, though, it is actually standard operating procedure. Only Sanders seems to think the normal thing to do once voters have made clear they dont want to nominate you is to continue campaigning anyway."
"When things are not called by their right names, what is said cannot make sense. When what is said does not make sense, what is planned cannot succeed. When plans do not succeed, people become uneasy. When people are uneasy, punishments do not fit crimes. When punishments do not fit crimes, people cannot know where to put hand or foot."