He waited long enough to make it official (!) but this is good news.
He was still teasing last week that he could go either way and had meetings with both Harry Reid & Mitch McConnell.
But considering that most Democrats and Independents in Maine voted for him, I think he knew they would be mad if he went with the Republicans, and he was rumored to be left-leaning anyway.
The Republicans ran nasty ads against him during the election, so perhaps that also ticked him off.