President Trump has imagined himself at the center of high-stakes nuclear negotiations since at least the mid-1980s, when he tried, unsuccessfully, to persuade the Reagan administration that it needed a New York real estate deal maker to lead arms-control talks with the Soviet Union.

President Trump is expected to overrule his top national security advisers and decline to certify the Iran nuclear agreement, according to people who have been briefed on the matter, a decision that would reopen a volatile political debate on Iran but is likely to leave in place the landmark deal negotiated by the Obama administration.