Manchin is up for reelection in 2018. Even before this election, he had a big target on his back. He is terrified about losing his seat.
Because of his fear of not getting reelected, he can be pressured into voting with Trump on some important votes — like Obamacare, supreme court justice, nomination, lowering taxes, etc.
If we can get enough other Democratic Senators, we might be able to prevent filibusters.
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We need his vote because of the RINO turncoats who will vote with the rats.