By itself, opposition to the deal may not sway many voters one way or the other, but it will make it harder for these candidates to get a hearing beyond their usual hawkish audiences. If this were an isolated case of opposing one deal, that would be one thing, but for all intents and purposes the position that virtually the entire field has taken is the rejection of diplomatic engagement itself. These candidates have had to adopt increasingly hard-line positions on Iran to placate the party’s ideological enforcers on these issues, and because of that they appear to everyone else to be actively hostile to any and all diplomacy. Combined with the GOP field’s lack of foreign policy experience, that is very likely to be a drag on whoever emerges as the nominee. If the eventual nominee could point to a record of good foreign policy judgment in the past, that could make criticisms of the nuclear deal a bit more persuasive. When criticisms are made by candidates with little or no relevant experience, they are more likely to be received as carping from people that don’t know what they’re talking about. When it comes to Iran and the nuclear issue, that perception will be grounded in reality.

—Daniel Larison
The Politics of the Nuclear Deal