WASHINGTON — In his most comprehensive effort to assure wavering Democrats, President Obama wrote in a letter to Congress that the United States would unilaterally maintain economic pressure and deploy military options if needed to deter Iranian aggression, both during and beyond the proposed nuclear accord.

The Aug. 19 letter, obtained by The New York Times, is addressed to Representative Jerrold Nadler, Democrat of New York, but is also aimed at other Democrats with concerns about the deal. For Mr. Obama, it reflects steps the administration could take outside the agreement. The president has repeatedly said that the deal reached by Iran and six world powers cannot be changed.

While many of the promises have been made before by Mr. Obama, Secretary of State John Kerry and others, White House officials say the letter represents the first time that the president himself has compiled them under his name and in writing. It commits explicitly to establishing an office within the State Department to carry out the nuclear accord.

In addition, Representative Adam Schiff of California, the ranking Democrat on the House Intelligence Committee, said that the letter expanded assurances that sanctions lifted under the nuclear accord could be reimposed piece by piece, not all at once, to keep Iran in compliance. Mr. Obama’s pledge to use the multinational commission policing the accord to block Iranian procurement of nuclear-related technology is new, as is the president’s explicit pledge “to enhance the already intensive joint efforts” of the United States and Israel in the region, said Mr. Schiff, a supporter of the deal.