“This would give Europeans even less breathing room to keep the J.C.P.O.A. alive until November 2020,” when the world would know whether President Trump is re-elected, Ms. Geranmayeh said. “It will be very difficult for the Europeans to hold it together.”

After conversations on Friday morning with officials in Tehran, she said that “people inside Iran pushing for the diplomatic process the Europeans have been advocating have lost a lot of ground.”

She said she expected the Iranian response to the killing to be on different tracks, potentially including military attacks on American targets in Iraq, Syria, Afghanistan or elsewhere; cyberattacks; or various other possibilities.

Part of the Iranian debate, she said, is whether to retaliate openly or through proxies in a more deniable way, which has been Iran’s practice in the past.