Who knows? An article in Politico says it depends on how the question is worded. That's probably true. Most people don't follow the details of foreign policy, so they react viscerally to questions. I suspect that most Americans would like to believe that an agreement limiting Iran's nuclear potential is good, but they have several decades of distrust of Iran as a counterweight.
The same was true of nuclear arms talks with the Soviet Union, but they worked out pretty well, as I note here.
In fact, public opinion will be an argument in the forthcoming congressional debate, but not really a reason deciding votes. Opponents have the luxury of saying whatever they want because they do not expect to prevail and thus be held accountable for the dangerous consequences of rejection of the deal.
Supporters are hesitating because they want to hedge against damaging revelations or surprises in the likely final vote. I'm sure many Democrats would like to be able to vote no to reassure pro-Israeli constituents if their vote didn't make a difference in the outcome. We'll see.