Joe Manchin is as liberal of a senator as you can expect from WV. A Republican senator would be worse.
Give me one tangible way, literally one. There isn't a single tentpole issue that Biden or the democrats can get passed because Manchin knows that if he votes for anything seen as progressing the democratic agenda he will get voted out. In many many ways I think he is worse than your standard republican in the senate, lets say his fellow West Virginan, Shelly Moore Capito. He doesn't stand for anything in the entire world other than getting reelected and maintaining his little cove of power, with the inability to do anything with it other than to have the capability to remain being called a US Senator. He's more responsible for the senatorial gridlock than Mitch the Bitch is. He's literally allowing both parties to not do a damn thing.
Sinema, I don't get at all, she literally has no chance at all to get reelected doing what she is doing. She's 100% getting primaried and if she survives that the wacko Arizona republicans will paint her as Carl Marx, Chairman Mao, Stalin, and AOC all rolled into one, and it will work. The only chance she had was to energize voters and she isn't doing that at all, she's cooked.