The Republicans don't trust Iran not to build a bomb.
Neither do the authors of this deal. The choices are:
(a) Do nothing and allow Iran to continue to be within 6-7 months of making a bomb, as they are now (b) Implement the deal and push that time out to 15+ years (c) Try even harder to negotiate a better solution (d) Stop them by force before they build it.
If you oppose choice (b), what other options do you propose?