Then why are they backing him?

They're not backing him, he's winning by unprecedented margins, and their zionist pet Janat Mills ate shit and dropped out, so it's a mixture of damage control and not giving up the kayfabe and endorsing the Republican candidate. Remember, this is Maine; Schumer can't hide there behind media noise or silence like he can in other states; it's too small and white, the Democratic party in that state would implode. So he gave Platner a mealy-mouthed endorsement

Also as an irrelevant aside, Platner somehow did the impossible and transformed the r/Maine sub into an anti-Zionist hub, a US STATE SUBREDDIT, I literally said out loud, WTF? I mean if this is the DNC's plan, I say don't interrupt the enemy