You and your team need to plan for every scenario and draft copy accordingly. Write different sets of copy for the good, bad and ugly.
But don't forget the best-case scenario: your campaign gets "over-funded."
This happened during our 24-hour campaign at Muhlenberg College when they reached their donor goal by 3:00pm and still had nine more hours to go.
We had to scramble to put together a new "reach" goal and write messaging that simultaneously celebrated the achievement and encouraged new donors to continue making gifts.