Second, if he is running for senator, he isn’t running as a spoiler 3rd party candidate for president.

FTA:

Still, even Democrats who belatedly accept that Manchin was never going to help them hang on to the Senate now worry about Manchin throwing his jawline into the presidential race as a third-party candidate—perhaps with the “No Labels” party, perhaps as a running mate to Mitt Romney—and hurting Biden’s reelection chances that way. I am not concerned. If anything, Manchin’s candidacy would be more likely to take votes from Trump than Biden. There just isn’t a lot of data to suggest that there are Republicans who are willing to vote for Biden because of Trump’s unique threat to democracy but would throw their democracy-saving vote away on a third-party candidate. And the kind of Democrats who hate Trump but also hate Biden are generally on the more progressive end of the spectrum and hate Manchin most of all. Moreover, there is simply no evidence that Manchin is well positioned to appeal to any distinct block of third-party voters. The myth of Manchin as some kind of independent iconoclast has always been more powerful than the objective reality of Manchin: a coal lobbyist nobody trusts who lives on a houseboat.

So even then, he’s likely to hurt Trump more than Biden as a political spoiler.