The psychotherapist and political academic Paul Hoggett has tried to support an ethic of welfare-based public service by linking compassion with solidarity. I am only in the early stages of learning about emotions in general, but just on personal experience I beg to disagree with Hoggett in part. I agree with him that compassion is more enduring than pity or indignation, but would see solidarity as something different. When I have felt compassion (for example, for rape victims) I did not in any sense join up with them. I remain other, even in my soul. The emotional connections are with anger and, yes, a sense of injustice, but not with solidarity.
Hoggett acknowledges that 'identification' of the self with another is commonly partial, but still overstates his case. In compassion I do open my mind to the other and try to learn from them (not tell them what is good for them) but I do not abandon my capacity for critical thought and simply join the other as a follower which is what solidarity tends to mean. Political movements which prize solidarity are apt to demand unquestioning allegiance.
The current urgent need to recruit more people for public services will have to accommodate a distinction between compassion and solidarity. For mental health services and social care compassion has to be in the forefront of motivation. For the military services, although compassion has its place (including in battle), solidarity is essential for motivation. There is a common problem for both in terms of reconciliation with a low tax economy, as compassion and solidarity each come from a source different from market based operations.