The promises were made. His future looked bleak. He'd been a turncoat when his party needed to show unity and he was going to be challenged and probably defeated in his primary. You simply don't lose when you are completing 30 years in your Senate seat. Yet, if you clearly demonstrate that you lack principle, you will lose not to a challenger from the other side, but to one in your own party.
So, he sought to grasp at a life-saver. He met with the other guys and they promised him what he asked for to jump ship. He jumped. Now he learns what the promises were worth:
You Knew We Were Snakes When You Crawled in the Nest
The bridge got burned and the ashes scattered. He's in with the pack and they've stripped him of the considerable power he had held even as a minority party member. He can't go back and they know they really can't trust him. So, they take him aboard to make the numbers work for the next 18 months and then pretty much insure he won't have the clout to make him attractive to knowledgable Democrat voters. They will put up someone they've got more confidence in.
And poor Arlen is finding the world to be a very harsh place.