In NV, Romney had 50.1% which would be considered in most math and stats classes to be close to winning a majority.
The math of the general election goes something like this:
A moderate candidate will have difficulty in a primary with the extreme wings of their party (say 40%) but will do well in the general election because that 40% will vote for the moderate over the opposition party. A more extreme candidate will have the base but may slip on the moderates. The view ignores the enthusiasm of the base which needs to be maintained for votes and manpower. IMO, both views are correct and you need the base enthused and someone who can approach the middle.
I am the rare Romney supporter here but I believe that if he wins the nominations he’ll have to some would class repair to excite the base including changes to policy, his cabinet choices, the party platform, and strong support for tea party / conservatives in congress. In short, he must be willing to fence himself in to win.