None of the motivations Manchin and Sinema put out there – or speculation in this comments section and elsewhere – make any sense. Either or both clearly have the power to tank everything (and take the blame) simply by switching parties. They obviously both love that power since ANY D senator has the same leverage, but they aren’t seeking anything in particular except, it seems, capitulation to their willingness to be a spoiler. As soon as they face Progressive pushback, they get all indignant!!
The only rational basis I can derive for Manchin’s and Sinema’s opposition to the vast majority of their self-selected party is that they think the D majority will be short-lived and they want to maintain some position of importance when Rs take control of one chamber of Congress or the other. So if Biden can show them solid polling that indicates that the Senate is going to shift more toward the Ds in 2022 (e.g., PA, WI, NC), making their obstinancy now obsolute in January 2023, that might make them get with the program lest the lose their limelight AND take the blame.