Proponents of the bill deny it would deprive Iran of the right to modest enrichment. They point to the qualifier “illicit” in the reference to nuclear facilities that must be dismantled, and they say the language on enrichment is intentionally vague to mollify both Republicans, who are reluctant to grant Iran the right to operate even a single centrifuge, and Democrats, who balked at signing on to a bill that would rule out all enrichment.

“There’s no language that says a centrifuge is prohibited or allowed,” said David Albright, an expert on Iran’s nuclear program at the Institute for Science and International Security, who helped Republicans and Democrats draft some of the technical wording.

The ambiguity, he said, reflected the fact that the lawmakers who sponsored the bill are “doing it in a bipartisan way, but they have disagreements on what the end state should look like.”

Mr. Albright, however, said the provision on ballistic missile testing could pose a problem. Proponents say it merely echoes prohibitions on such tests that are in United Nations Security Council resolutions — resolutions that Iran must confront in the next round of talks. But Iran’s missile program was not part of the interim deal, and introducing it now, Mr. Albright said, would inject a combustible element into an already fraught negotiation.

The requirement that Iran not engage in terrorism against Americans seems self-evident: The United States is not about to make a deal with a country that attacks its citizens. But the language is vague on the time frame — Iran was certainly guilty of terrorism against Americans in the past — and broad in its scope, including Iranian proxies like Hezbollah.

As if to illustrate the problem, Iran’s foreign minister, Mohammad Javad Zarif, placed a wreath this week at the grave of a Hezbollah commander, Imad Mughniyeh, who was accused of being a mastermind of the 1983 bombing of the Marine Corps barracks in Beirut, Lebanon. Any future attack by Hezbollah would be grounds to cut off diplomacy with Iran.