At Politico, Gary Sick writes that, if Iran’s Foreign Minister Mohammad Javad Zarif fails to capitulate to the demands of the United States and a nuclear deal isn’t reached, it would play into the hands of Iran’s hardliners. As well,
… the failure to reach a deal by Nov. 24 would in all likelihood have a second effect that would compound the problem: weakening the external leverage that the United States could bring to bear on Iran. The primary leverage that the U.S.-led side has brought to the table is the international sanctions regime that has limited Iran’s energy exports and choked off its access to international financial networks. But those are not U.N. sanctions; they rely primarily on Washington’s ability to persuade or pressure companies in countries whose governments do not endorse those sanctions to refrain from trade with or investment in Iran, under threat that noncompliance could result in their being shut out of the international banking system.
However
… if Iran is internationally perceived to have made a good-faith offer of compromise to no avail, the dynamic could change. Washington might no longer have that leverage.
Sick notes that
One of the great triumphs of President Obama’s diplomacy on Iran has been the ability to hold together the very disparate group of negotiating partners—the United Kingdom, France, China, Russia and Germany. … China and Russia, as well as the European powers, would welcome an agreement that removed the constant threat of a military confrontation over Iran’s nuclear program, and gave Iran a potent incentive to become a responsible international citizen. To that end, all parties have been willing to back what has increasingly become a bilateral U.S.-Iran negotiation, tacitly recognizing that no agreement on the issue is possible without full U.S. involvement.
But (emphasis added)
… deference to the United States, whose control over crucial international banking mechanisms gives sanctions their bite, is not unqualified. If the United States should reject what is perceived to be a reasonable Iranian offer, there are growing signs that the coalition might begin to fray.
Furthermore
Most foreign firms are unwilling to risk Washington’s wrath as long as negotiations are underway, but that reluctance is likely to evaporate if these corporations come to believe the United States rejected an Iranian offer acceptable to their own governments.
Worse, as Julian Borger writes at the Guardian:
Failure would be a heavy setback for both Obama and Rouhani. But there are worrying signs that Rouhani is making preparations for a Plan B, that would help him to survive collapse in Vienna or its aftermath. That involves rhetoric, repeated by Zarif on arrival in Vienna, blaming the West in advance for ‘excessive demands’. It also involves signing deals and memoranda of understanding with Russia and China, as an alternative to the full reintegration in the global economy Tehran was hoping for.