Claiming it falls under Obama’s “executive prerogative.”

Via WSJ:

The Obama administration collided with Congress Wednesday over Iran sanctions, and both sides were girding for a brawl that appeared likely to stretch through the year.

Lawmakers from both parties vowed at a Senate hearing to press ahead with legislative plans for new punitive measures—over President Barack Obama ’s veto warnings. Top administration officials, meanwhile, disclosed at the hearing that the international talks over Iran’s nuclear program may extend beyond an end-of-June deadline. That would be the third such extension. […]

Deputy Secretary of State Antony Blinken said the White House doesn’t view an agreement with Iran as a treaty that requires Senate approval, but a matter of “executive prerogative.”

Jim Geraghty points out the section of the Constitution, Obama is going to trample this time:

Article II, Section 2, clause 2, U.S. Constitution:

[The President] shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur.