The whole letter is worth reading (it's only a page), but I particularly liked this part:
In moments of concern and crisis, Catholic tradition through the centuries has unfailingly urged the faithful to turn to the spiritual aids of prayer and fasting in order to draw closer to our Lord and His will. Through these deliberate actions, we communicate a desire to avoid sin and unify ourselves with that which is right and good. I firmly believe that, working together while open to God's wisdom, the citizens of our nation can respect the dignity of each human person both in law and in practice.
Although the law of fasting binds persons age 18 through 59, anyone can voluntarily fast. Fasting allows a person to eat one full meal as well as two smaller meals, which are not to equal one full meal. In addition, I ask you to consider offering a Rosary, prayers at Mass, time spent in Eucharistic Adoration or other forms of prayer for this important intention. Through our fasting and prayers, we ask the Lord to lead the hearts and minds of our nation's leaders as they make crucial decisions concerning the protection of life.
Mother Mary, pray for us!
*I don't think I've ever seen this used to declare a day of fasting, but this is a very worthy time to start.
Reference: master-of-pentagram.blogspot.com