Because neither party will advance a bipartisan bill (I.e. one that is appealing enough to both parties to attract a majority of votes), Democrats need all 50 o

Because neither party will advance a bipartisan bill (I.e. one that is appealing enough to both parties to attract a majority of votes), Democrats need all 50 of their senators to vote on their bills, which lets them pass via a tie-breaking vote from Harris. Manchin is the least in agreement with his party about their bills, so he has a lot of power as they cannot attract a 50th vote from the other party so need to appease him.