Congress passed H.R.1191 – Iran Nuclear Agreement Review Act of 2015 on 5/22/2015 and Obama signed it making this public law 114-17 PDF

This bill requires Congress to pass a resolution of disapproval to reject the deal, which would almost certainly be vetoed by Obama. Then, Congress would be required to muster votes from two-thirds of each chamber to override the veto. All the president has to do to force a bad Iran deal on America is keep 34 senators of the Democratic Party or 145 members of Congress on his side which is can do with little effort.

What this legislation did was turn a 67 vote to pass into a 67 vote to stop.

There was no reason to pass this bill so one can only assume the Republicans wanted the deal just as badly as Obama did.