It will not matter to M&S what value the seat has to Democrats. They're trying to stay in office. If Biden can't get his program passed as he wants it, that's his problem - though I bet he will, in different form.

There is no reason either of these two should give a damn about it, except for retaliation by the insane party zombies chasing them into bathrooms. That said, they still help the party by giving it control over the senate.

Now, are they helping themselves? Manchin, very likely, yes. Sinema is in very good position for a general election, but may face trouble in a primary. I think she realizes that Arizona is a lean-red state, and that this is the only way she can have a truly safe seat in the general. But she may underestimate the lunacy of the primary voters. It will remain to be seen whether her actions help or hurt her in the end. She may go work for a pharmaceutical company or something.