In this sentence - Liberal Democrats flock together - what does flock together mean?
It means they work together, and even play together (I'm sure at least some politicians know how to play) because they share the same ideology and interests.
It's derived from the age-old idiom "birds of a feather flock together". "Birds of a feather" refer to birds of the same kind and color because they are coated with the same type of feathers. Out of natural instincts they stick to each other, feed together, play together, fly together and even fend for each other in face of mutual enemies.
Similarly, when used to describe people, birds of a feather flock together points to the fact that people of similar hobbies and interests tend to congregate and go in for the same causes, political or social, recreational. That's why it is often said that you can check out a person by getting to know the kind of friends he keeps.
Like it is in the animal world, people who "flock together" may just as well stick up for each other in situations of difficulty.
This is not always applicable politically, though. Politics often requires that one shift to the winning side in times of political movements and power change. Political circles, therefore, are not so much known for sticking up for a colleague than for backstabbing him instead, sometimes along with a public announced severance of all ties with him when a former ally is on the way down and out.
There are exceptions of course. Every once in a while, there is story told of political rivals staying loyal to their friendship in spite of their political inclinations. And that's as it should be. Friendship is greater than current politics. Even then, however, two political foes don't FLOCK together, at least not in public - that would mean political kiss of death for both.
The word 'flock' originally is applied to animals, or birds, in crowding groups and packs. There typically is the flock of the weak and meek sheep, so thick and tight that it's hard to distinguish them individually. Thus, when used to describe people, "flock" emanates similar connotations, emphasizing on the collective group rather than each individual.
This passage from the Strange Case of Dr. Jekyll and Mr. Hide (Robert Louis Stevenson):
Thereupon the servant knocked in a very guarded manner; the door was opened on the chain; and a voice asked from within, "Is that you, Poole?"
"It's all right," said Poole. "Open the door."
The hall, when they entered it, was brightly lighted up; the fire was built high; and about the hearth the whole of the servants, men and women, stood huddled together like a FLOCK of sheep.
At the sight of Mr. Utterson, the housemaid broke into hysterical whimpering; and the cook, crying out, "Bless God! It's Mr. Utterson," ran forward as if to take him in her arms.
"What, what? Are you all here?" said the lawyer peevishly. "Very irregular, very unseemly; your master would be far from pleased."
"They're all afraid," said Poole.