The Maryland Democrat is the third Senate Democrat to come out against the deal.
The Office of Congressional Ethics found “substantial reason to believe” rules were broken.
The announcement comes amid speculation the N.D. senator could run for governor in 2016.
Booker is the 35th senator to back the deal.
Scott Wong 09/03/15 12:52 PM EDT
"I don’t work for John Boehner. He works for me," Rep. Lynn Jenkins said.
The Minnesota Republican has announced he will not seek reelection.
Warren declined to say whether she discussed a joint 2016 presidential ticket with the vice president.
The measure to reject the president's nuclear agreement had been expected the following week.
The House vote to reject the nuclear deal is slated for next week.
The secretary says administration will work on regional security following Iran deal.