The Mexican War made it difficult to keep the new political parties intact. Because the north saw the war as an attempt to acquire more slave land the politicians began to side more with their section than with their party. When David Wilmot tried to pass a rider on an appropriations bill that outlawed slavery in any new territory, party unity completely collapsed as everyone voted sectionally. Slavery suddenly became the only important topic and the party system as they knew would begin to fall apart.
Historically Speaking 