What part of "best progressive on the ticket" don't you understand? I'd absolutely hate him for his stands on the environment, opposition to abortion, immigration policies, votes for Kavanaugh and Rolling back Dodd Frank. I'm sure I'd be knocking on doors and showing up at Paula Swearengin rallies if I lived there.
On the other hand, Manchin supports unions, retiree pension plans, shoring up the ACA, continuation of Social Security, Medicare and Medicaid also reinstating net neutrality. With Manchin we get 40% of the time voting against Trump. With the Republican we get less than 10% of the time voting against Trump. Manchin gets my vote in the fall IF I lived in WV.
It's simple arithmetic. Even you should be able to see that 40% > 10%
That said, I live in Oregon. Wyden votes against Trump 85% of the time and Merkely votes against Trump 90% of the time. I voted for both. I'm much more satisfied with the idea of them as my senators compared to Manchin. Manchin couldn't win as a Democrat in Oregon.