Senator Charles E. Schumer, Democrat of New York, said he intended to “go through the agreement with a fine-tooth comb.”

Mr. Schumer is usually a strong supporter of President Obama and his policies, but he has stepped carefully on the Iran nuclear deal, and he backed the bill that gave Congress an opportunity to review the agreement. Mr. Schumer has faced conflicting pressures on the issue from constituents and donors in his home state and factions within his party.

“Supporting or opposing this agreement is not a decision to be made lightly, and I plan to carefully study the agreement before making an informed decision,” Mr. Schumer said in a statement.