The Social Security and Medicare trust funds are flat broke. They have more outlays than receipts.
The US is about to spiral into a deflation depression. Massive deficit spending (debt-money creation) is the only thing that has pretended to keep us out of it thus far. The Federal government relies on taking excess receipts from trust funds and purchasing US Treasury securities with them. That option does not exist anymore until a new HUGE "trust" fund is created. That's what the ACA is all about.
In other words, the Senators know full well that we face economic ruin very soon if ObamaCare is not pushed forward. They know that they can't negotiate out. Our nation of debt addicts has run out of time.
Bingo