That explains it

Kathryn Jean Lopez noted on The Corner The clearest reason the Dems blocked the Immigration bill in the Senate:

In private as well as public, Reid and Sen. Charles Schumer, D-N.Y., who heads the party's campaign effort, said they did not want to expose rank-and-file Democrats to votes that would force them to choose between border security and immigrant rights, only to wind up with legislation that would be eviscerated in future negotiations with the House, which has passed a bill limited to boosting border security.

That explains it. I thought they were just being Democrats, opposing everything, even though the "compromise" was almost a complete surrender to them.