Does McConnell actually believe the filibuster will remain, or did he just think that he wouldn't be able to get it guaranteed in the resolution so continuing the fight was pointless?
No it's a political game, Mitch wants them to do it so they can use it as talking points. The deficit isn't a good talking point anymore with voters, so he'll let them go this route. DC Statehood will be one, killing the filibuster and I think eventually packing the courts will become a thing. It'll all be things to scream and b**** about for the GOP in the midterms. I don't think the GOP regains the Senate but the Dems really need to have a good strategy for the House, that seems much more likely to be taking over.
