Which two GOP candidates would create the longest primary?

[Never]

If two GOP candidates are making a very long primary, it would either mean that they are mediocre and not capable of winning a high amount of support, or that they are both very strong candidates who appeal to different wings of the party.

The weakest candidates who could plausibly make it out of IA/NH in 2016 seem to be Paul Ryan and Mike Huckabee, who would both probably have difficulty resonating with voters outside of their very narrow range of support. While Ryan is fiscally conservative, he doesn't seem to relate with the Tea Party or values voters, while Huckabee could carry socially conservative values voters and trouble with economically conservative Republicans in the Northeast and Midwest. Ryan would probably win most of the Northern states outside of IA, but he might have trouble in the Rust Belt if Huckabee is running a very populist campaign. Huckabee would surely win most of the South. The Western states would probably be fair game, with Huckabee and Ryan splitting the states in that region. Huckabee would probably prevail, since the GOP is a somewhat Southern-centered party, which might help him as the primaries go on.

It seems like the strongest candidates are Chris Christie and Rand Paul. It would appear that Christie is a traditional Republican, while Paul seems to have a new direction for the Republican party. While I understand that Christie and Paul are very different from Nelson Rockefeller and Barry Goldwater, there are some similarities between these pairs of candidates that are hard to miss. Rockefeller and Christie seem/ed to be favorites of the party regulars, while Goldwater and Paul appear/ed to be motivated in rallying supporters to their causes.

A long primary could be good or bad for the Republicans, but I think that it would be hard to confidently tell which two candidates would be most likely to have an extended showdown in the primaries until late 2015.