The US Senate has approved the Iran Nuclear Agreement Review Act, which if passed by the House of Representatives would require Congressional approval for any agreement with Iran in respect of its nuclear program. At present, the President has the authority to lift US sanctions unilaterally, but under the proposed legislation Congress would have 30 days to review any proposed agreement and would then be able to enact a joint resolution supporting or opposing it, or take no action. If Congress opposes the agreement then the President may veto their resolution, which would be subject to a 2/3 vote to override it in Congress. If a vote to override the veto is successful then the joint resolution would prohibit the President from lifting sanctions.
The legislation could complicate US negotiations with Iran, which has publically insisted on full and immediate relief from sanctions – something that would be all but impossible under the proposed review process. It could also lead to a situation in which the UN Security Council has voted to lift sanctions while US sanctions on Iran remain in force.