I suppose it would have to go something like this: a fairly orthodox (though by no means authoritative) genre of Marxist thought insists that revolution can be triggered by advancing capitalism to the point of crisis. In practice, this would involve the abolition of the democratic welfare state, the consolidation of power into the hands of the bourgeoisie, absolute privatization, the heightened immiseration of the proletariat, and so on. Make all of this happen, and capitalism becomes completely unsustainable and digs its own grave, as Marx put it.
If you suppose that it's possible to make this happen through electoral politics, and calculate that we would all be better off if this happened sooner than later - again, all entirely orthodox Marxist positions, though not at all authoritative - then it follows quite directly that you would want to elect the absolute worst capitalist candidates you possibly can.
That makes the case for nominating Hillary Clinton over Bernie Sanders completely straightforward. There are plenty of Marxists who (correctly, I think) insist that Sanders is himself a standard welfare capitalist, but I don't think one can defensibly argue that he's a worse welfare capitalist than Clinton. If we had one of our decennial economic crises under Sanders, for example, it seems probable that he would take the opportunity to strengthen the welfare state, advance various financial regulations, and perhaps even break-up and/or nationalize key banks; Clinton, meanwhile, is far more likely to react with austerity measures and less ambitious regulations (if not de-regulations).
Again, I don't think there's anything particularly heterodox or implausible about any of this. I personally suspect that the certain harm of a Hillary regime is not an ethical tradeoff for the uncertain prospect of possible revolution, but this is mostly a judgment call. The more significant challenge for Clinton supporters is that they can't maintain this argument against Sanders in the Democratic primaries and then justify a vote against Trump in the general election; still, if you're a Clintonite who wants to call yourself a socialist, you have better arguments at your disposal than the ones you're making.