In 1856 R.W. Emerson attributed solidarity to a general knowledge or affective continuity amongst the English of all classes: "One secret of their power is their mutual good understanding... They have solidarity, or responsibleness, and trust in each other."
Emerson was wrong about the affective unity of the English, but there's something tender in the phrase "solidarity, or responsibleness" that's worth pursuing.
Solidarity is an affect, a thing you have a sense of. It's in the atmosphere, it's being tested all the time, and when one is confident about it it's a huge relief to feel it--whether from people or just as a sense in the room when you're there, at a meeting or a protest, or just a conversation. It's something patriotism cultivates: political movements too.
It communicates an intensity of political reciprocity.
Then there's solidarity as in the Polish movement, which itself extended from the Marxist ideologeme. Solidarity there expresses a mobilized class interest, the sense of groupness when the class becomes "for itself."