Does an understanding of politics as transformative action not clash with one of it as a practice of belonging?
Certainly not. The whole Marxist tradition conceived of class consciousness as a practice of belonging.
However there are problems the Marxist tradition never solved. You want a universal working class conscious of its own transformative agency; but you will not be able to describe this class in terms concrete enough to address any member of it in particular. No one can, those days are over, the language does not fit the times.
When the *industrial* working class could still be conceived as a revolutionary subject, such a description was possible. Marx and Engels did it brilliantly, by spending years debating their ideas directly directly with the workers. But after the crisis of the 1930s, all capitalist states recognized the danger represented by the working class and made extraordinary efforts to integrate the industrial workers to capitalist practice, first through wage bargaining, then through benefits, then through a variety of cultural and even military appeals, culminating in the current situation where industrial workers are recruited to fascism with anti-immigrant nationalism and the vague promise of industrial jobs.
This doesn't mean there is no transformative potential left in the industrial working classes. But they can't hold the place of a universal political subject,and the class you are looking for - singular, concrete, conscious of itself and ready to act - is not solely defined by work anymore.
In fact, the focus of the state on work and the workplace encouraged anyone who cared about class to look outside the factory and even the wage relation for the inequality and injustice of capitalist societies. Because those societies now focused as much on consumption - and more broadly, on what Marxists call "social reproduction" - as they did on production, direct oppression exerted by the capitalist state and by the forms of social reproduction that it mandated could be found in many different places. Identity politics emerged as a way of naming those sites of oppression, and even more importantly, as a way to gain transformative agency through the consciousness of belonging to an oppressed group.
The upshot is, that if you wanted to redo Marx and Engels, you would have to start not by rereading their books and their tradition, but by taking new ideas of both oppression and transformation down to the places where identity politics is debated, and giving those new ideas a go.
Now, this all does not mean everything is fine with identity politics as it is practiced today. Certainly just abandoning the question of work is the wrong path (but no one serious does it, so I don't know what the problem is?). A new universal is definitely lacking, and much can be learned from the attempts to conceive a universal working class. However, it does mean that you can't just diss off identity in favor of some supposedly correct concept which you have totally dehistoricized, particularly by ignoring the dialectical negations to which it was subject. No one will take you seriously if you do. Today, pretty much every "return to Marx" is a return to some nostalgic and usually privileged self, alone even in the typically tiny groups, trying to convince themselves that their pure idea from the past can overcome everything that has happened in global society since 1968.
Don't get me wrong, I'm not saying "Marx to the trashcan." I'm just saying that if you do go to the barricades, you will not find a universal working class, and the language with which you seek to invoke or catalyze one, will remain empty and useless. Doing real politics is far more demanding than most of us can handle. The "back to class' posts in this thread are so vague, so nostalgic, so empty, that they do not come anywhere near the goal.
What we are missing is a theory of social relations in the future. To be transformative it will have to be inclusive, combative and aspirational, attuned to a possible life beyond the dead-ends of the twentieth century.
best, BH