There has been criticism of both, and I've heard far more outrage at Manchin than Omar recently.
What would be Bidens reasoning in wanting his own agenda to fail?
But if you need something more concrete, you mention getting both out through primary challenges. There are already calls for this to happen with both Manchin and Sinema. You said to give Manchin something in exchange for support in the removal of the filibuster. Biden lowered the cost of his infrastructure bill by 1 trillion. What do you think that was about?
The fact is, Manchin is a conservative Dem in a deep red state. The hate he is getting from other dems probably makes him more popular in his state, thus gives him an incentive not to budge. How do you leverage this?
When it was announced during the 2020 that the dems retook the senate with the thinnest of margins, I sort of expected something like this to happen. It's disappointing yes, but I blame the defectors and not the party as a whole.