<p>Senate Democrats who've been wobbly on the Iran deal are breaking President Barack Obama's way. </p><p>Three key Democratic holdouts threw their support behind the nuclear accord with Iran on Thursday — one day after Obama clinched enough votes to guarantee Congress can’t kill his agreement.</p><p>Sens. Heidi Heitkamp of North Dakota, Mark Warner of Virginia and Cory Booker of New Jersey formally announced their backing.