Sen. John F. Kerry, the Democratic challenger, has suggested that the U.S. should back a deal to provide Iran with nuclear fuel, something the Bush administration has so far refused to support.
Despite the absence of clear evidence, U.S. accusations against Iran have gained wide acceptance in recent months.
The main reason is that the suspicions do not rest as heavily on U.S. intelligence as they did in the case of Iraq's alleged nuclear program. International concerns about Iran are rooted in information uncovered by IAEA inspectors and described in six detailed reports.
"Evidence gathered by the IAEA makes a circumstantial case that is much stronger than the case that Iraq was restarting its nuclear program," said George Perkovich, a proliferation expert at the Carnegie Endowment for International Peace in Washington. "The only thing missing in Iran is a weapons design."
Barring a last-minute surprise, insiders said, inspectors have no concrete evidence of a weapons program or new instances of concealment. Still, the report is expected to conclude that too many big mysteries remain for inspectors to give Iran a clean bill of health.
"There are many reasons for worrying about Iran's intentions, but you have to be careful jumping to saying there is a weapons program or not," one of the European diplomats said.
The most pressing concern is identifying the origins of small amounts of weapons-grade uranium and low-enriched uranium found at four locations during the last 18 months.
Iran says the material came from contaminated centrifuge components bought on the black market. Abdul Qadeer Khan, a Pakistani scientist who helped develop his country's nuclear weapons, has confessed to selling components to Iran, Libya and North Korea.
In its September report, the IAEA said it was plausible that some of the enriched uranium came from Pakistani parts. But some concentrations were larger than simple contamination could explain, and not all of it was necessarily from Pakistan, the agency said.
Despite the high priority, inspectors have made no progress in answering the question because Pakistan refuses to cooperate fully, several diplomats familiar with the inquiry said.
Pakistan provided some data and said pointedly that not all of the enriched uranium came from its program, but diplomats said the Pakistanis had not allowed inspectors inside their nuclear plants to take samples.
The IAEA wants its own samples so independent laboratories can determine conclusively whether the material found in Iran matches enriched uranium produced by Pakistan.
ElBaradei said in late September that Pakistan had refused to let the agency question Khan. U.S. authorities also have been unable to interview the scientist. A Western diplomat complained that the Bush administration was not pressuring Pakistan to allow the IAEA to take samples or interview Khan.
The failure to trace the contamination leaves open the possibility that Iran produced weapons-grade uranium at a secret plant or bought it from an unknown supplier, diplomats said.
"Most of the traces are from Pakistan, but if some of it is not, then it is a serious issue that raises the possibility that Iran produced it," one of the European diplomats said.
Another issue is how much work Iran did on advanced centrifuge machines, known as P-2s.
In October 2003, Iran submitted a multivolume document to the IAEA that it said represented the complete history of its nuclear activities. But it began to unravel three months later.
When Libya decided to give up its clandestine nuclear weapons effort, it turned over information and technology to the IAEA and the U.S. It became clear that, like Iran, Libya had bought nuclear technology from Khan's network.