The problem with a big tent party is that it becomes very difficult to get anything done.

Parties based on relevant lines form because like-minded people tend to group to increase the power of their vote. Therefore, conservatives on an issue will group with other conservatives, liberals on an issue will group with other liberals, and so on. This is natural and essential to politics.

It's true that not every issue is shared by every member of the party, but in the end all elections and all votes must come down to a value decision. I'm sure most people here will agree on some issues with every one of the candidates seeking the nominations of the Democratic and Republican parties, and most of you will also disagree with some of their positions. But unless you're running yourself, it is going to come choosing the person who you agree with most consistently.

Now, as for the future of the Republican Party, I don't think there's any doubt that a centre-right party will exist in the United States, no matter what the fortunes of the Party itself. But the fact that the Republican Party has held hegemony for so long, based largely on the votes of conservative evangelicals, demonstrates that as a constituency they are not terribly weak, and do not seem to be going away very quickly. By the time they are weak enough to be a liability, they will already have been replaced with another strong conservative group. This is, again, the nature of politics.