Why have the Democratic and Repulican parties been so durable so as to maintain existence since the Civil War? Explain
It was also very true in the Gilded Age, and today, that these two parties are not just groups of individuals with members and political viewpoints, but they are a concentration of money and resources that third parties find it very difficult to compete with.
Party leadership has been quite adept, most times, at playing themselves off as viable alternatives to one another while the opposing party is in power, even if there is virtually little difference between the two ideologically. They are both moderate parties on average, looking to appeal to the mainstream of America because this is how you win elections.
Most third party movements have started out of a desire for reform or in the middle of crisis, and they typically fade due to fragmentation and lack of funding.
There is no single correct answer to this. However, to me, the most likely answer is that the two parties have survived because of the US electoral rules.
In the US system, it is very hard for a third party to become important. In fact, it has not happened since the issue of slavery and possible secession caused the splits in the Whig and Democratic parties before the Civil War. This is because we do not allow for proportional representation or anything like that which would allow a third party to start small and grow gradually.
Because of these rules, the two parties have been able to adapt to changing circumstances. When new issues arise, they are simply absorbed into our two party system. One party takes each side and nothing really changes. I believe that this is because of our electoral rules.