Partisian vote against Iranian nuclear deal would damage our allies confidence in U.S. leadership
A "nay" vote by Republicans against the Iran deal can have grave consequences even if they cannot override a presidential veto. Division, mostly along party lines, is never cost-free on national security issues. Countries hedge against American leadership even when U.S. foreign policies have a broad base of domestic support. They hedge more when domestic divisions convey that U.S. policies are not sustainable. Nuclear proliferation can be managed with this agreement; that becomes much harder if it unravels.
Capitol Hill has become a theatrical, partisan battleground where lawmakers can cast votes in the confident expectation of coming up short. Republicans can vote repeatedly against Obamacare without having to deal with the consequences of leaving millions of Americans without coverage because they can't override a presidential veto or the Supreme Court. Democrats can vote in large numbers against fast-tracking the Trans-Pacific Partnership because they know the White House will still be able to cobble together a majority.
Voting against the Iran agreement is different. A deep partisan divide would send all the wrong messages to U.S. friends, allies and adversaries. What we need is a bipartisan strategy to cope with the deal's downside risks and shore up our partners in the Middle East that will be affected by the lifting of sanctions on Iran.
If the U.S. congress rejects the nuclear deal with Iran, it will have multiple negative reprecussions for U.S. interests and security. The most likely impact of U.S. rejection of the deal will be a gradual collapse of the sanctions regime as our partners have no interest in reopening negotiations with Iran and are already starting to renew trade relations. Additionally, rejection of the deal will damage U.S. credibility as a global leader and its diplomatic capital for many years as the U.S. was critical in forming the consensus for the deal in the first place.