Regarding Manchin, from what I've been able to gather he has never opposed the caucus on a vote that really mattered.
Not so fast... His voting record on environmental issues is abysmal, and he now stands to chair the Energy and Natural Resources Committee.

https://projects.fivethirtyeight.com...e-manchin-iii/

Voted against S.J.Res. 53 which would have rescinded Trump's relaxation of emission standards for power plants; voted for the appointment of Andrew Wheeler (a former coal industry lobbyist) to head the EPA (W. Virginia is the 2d largest producer of coal in the US); voted against H.J.Res. 36 which would have relaxed regulations on methane emissions from oil and gas industry operations (a +1 for Manchin); voted for the approval of Scott Priutt as Administrator of the EPA (who later had to resign amid a slew of ethics scandals); and who voted for nearly every one of Trump's government appointees.

A mixed bag to say the least, but his stance on energy production is quite clear----he won't do anything to piss off his corporate donors in the coal industry. If he gets to chair the Senate Energy and National Resources Committee, it will be extremely difficult if not impossible to get any clean air proposals through the Senate even if the Dems pick up the two seats in Georgia.