A party realignment is when a political party realigns itself to new principles, and usually occurs after a major event that may alter the party's existing principles. An example of party realignment was seen in President Franklin Roosevelt's New Deal.
With the New Deal, the Democrats realigned themselves to back big government, to help those who were experiencing the greatest financial distress and to believe in the necessity of pouring money into domestic programs. This realignment was a response to the Great Depression that the country was experiencing. Nearly 50 years later, the party is thought to have the same values. It is possible that the party will experience a realignment in the future.