Final answer:
Third parties, unlike demographic shifts, key issues, and party strategies, do not represent a major contributing factor in party realignment within the U.S. two-party system.
Step-by-step explanation:
A major contributing factor in party realignment refers to the shifting of party allegiances within the electorate. Certain factors like demographic shifts, changes in key issues, and changes in parties' strategies are known to contribute to this realignment. However, third parties, despite bringing attention to specific issues and influencing elections by splitting votes, do not typically result in major party realignment within the two-party system of the United States. This is attributed to factors like the Electoral College, voter identification with major parties on a national level, and the historical lack of support for dissenting groups that might back third parties.