The Maryland Democrat is the third Senate Democrat to come out against the deal.
The announcement comes amid speculation the N.D. senator could run for governor in 2016.
Booker is the 35th senator to back the deal.
Warren declined to say whether she discussed a joint 2016 presidential ticket with the vice president.
The secretary says administration will work on regional security following Iran deal.
Jordain Carney 09/02/15 10:26 AM EDT
The Maryland Democrat's support all but ensures the agreement will survive in the Senate.
Coons said Biden deserves the "time and the space" to make a decision on whether to run.
The President is one vote away from securing a major foreign policy goal.
Julian Hattem 09/01/15 01:37 PM EDT
Democrats are now just two votes away from upholding Obama's veto.
The top Dem on the Foreign Relations panel said he will announce his decision "soon."