If the Republicans want to remain relevant, they must accept anything brought to the table.
This is all the more reason to raise the stakes, because they're going to be desperate after the holidays. Rather than putting Chained CPI on the table remove it completely and put raising the cap on the table. Keep all the other stuff, though. Poverty exemption, close the tax loopholes.
Raising the cap only affects the top 5% of the population. It would be called a tax raise but you just point out it keeps Social Security solvent and it doesn't affect the "bottom 95%." Keep pushing that.
If they pull their shit again and don't accept the offer, they will look even worse than they do now, so it is in their interests to accept. I think we could come away with this a big winner in a huge way.
Chained CPI off the table.
edit: oh yeah and for the concession lower the corporate tax rate to 28% since the effective rate is only 12% thanks to the loopholes. It would still be a tax hike on large corporations because businesses and individuals can't actually take advantage of the loopholes, only big corporations can.