The Iran Deal does not have the weight of a treaty that was negotiated by the executive branch and ratified by two-thirds of the Congress, as required by the U.S. Constitution. Because Obama lacked the support to make it a treaty, he negotiated it as an executive agreement. The benefit was that it allowed him to secure the deal, but the drawback was it left it open for revision or termination. He was aware of all of this, and went ahead with his deal anyway.
Congressional Republicans were emphatic while the deal was being negotiated that anything negotiated by the Obama administration without the approval of Congress should effectively be seen as just that — Obama policy.