Republicans who oppose the deal will be tarred as traitors.
Obama has decided that two wildly unpopular policies, one foreign and one domestic, will be the final legacy of his wildly unpopular administration. The domestic policy is gun control. The foreign policy is Iranian nukes. While Americans will be disarmed, Iran will be getting ready for its ballistic missiles.
Ramming through wildly unpopular policies is what this administration does best. More than anything else, this administration will be remembered for the mix of bullying, smears, pop culture distractions, outright lies, bureaucratic sabotage and blatant lawbreaking with which it achieved its policy goals.