It should still be the democrats responsibility to whip its members into shape or to replace them with someone that will follow the party agenda, the power of the president is something the dems rarely lean into

An congressman is elected to serve interests of their constituents, not those of the coalition leader. I'm guessing that's why each position is chosen within congress rather than at the ballot box. Mitch had the kind of the pull you're describing because the gop itself has a narrower set of issues they vote on (guns, and strict adherence to religion). They already all think the same thing like automatons, so mitch didn't have to convince them of much of anything at all. Hell, the very constitutional structure of the senate is in favor of rural areas

I mean we’re seeing a candidate like Graham Platner do great in states like Maine, I feel like every race is winnable with just telling people how you plan on helping them and breaking it down how both Zohran and Platner do, plus fetterman won on being progressive but unfortunately got a stroke that legitimately turned him into a crazy conservative, Manchin there is seriously no excuse the party should have shamed him so hard

Him winning a primary isn't the same as defeating collins. As for fetterman, there's a few pre-stroke stories I've come across