I think he's more like Linda Murkowski, Susan Collins could be counted on to cave under pressure. It's an inevitable consequence of mathematics that the smaller the balance the more swing votes count. You get power but you're the one on the hot seat.

Every Democrat is essentially a swing vote. With a 50/50 split and the Vice-President as the deciding vote, any Democratic Senator effectively becomes a potential swing vote.

I suspect Manchin was sending a signal to the Progressives. Also Manchin represents a state Trump took by 40 points.