Manchin’s merit is a myth. He is a complete Democrat.
I think Manchin either voted against the DREAM act and various "immigration reform" (=amnesty) bills or abstained from voting. In contrast, liberal Republicans like McCain and even some so-called "conservatives" like Rubio spearheaded those bills.
The point isn't so much that Machin is good as that there are many terrible Republicans who may as well be Democrats on a lot off issues. This is why many establishment Republicans either covertly or overtly supported Hillary and did their utmost to sabotage Trump in the last election.