There were two bills, the first being Kirk-Menendez, which never had a chance of receiving the 67 votes needed to override the veto.
Into that stepped Corker with his bill which could, depending on the merits of the final agreement, get the 67 votes for override. Corker's bill also contained the pre-emptive sanctions found in Kirk-Menendez, in case Iran walked before the final agreement was reached.
It was either Corker's bill or no bill.
Corker's bill also met the requirements set out by Mossad.
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The Corker bill undermines the Constitution. Under its terms, Obama can lift the sanctions against Iran unless both houses of Congress reject the deal by the two-thirds majorities necessary to override a presidential veto