I think they are concerned about November. If Obama waits for Congressional recess in a week, and then "acts because Congress won't", it might cause a major midterm backlash. After November, that would be a different story for them.
If their constituencies were writing them letters informing them that they had better grant amnesty or else, they would be singing a different tune. That isn’t the nature of the letters that they’re getting however. I think the Democrats are divided now that they have all seen how Obama’s immigration game is developing.