AS NEGOTIATIONS with Iran on its nuclear program resumed last week , President Obama reiterated his opposition to new sanctions legislation. The legislation, which has strong bipartisan support, could “undermine the negotiations” and isolate the United States from its allies, Mr. Obama said Friday. “Just hold your fire,” he urged Congress, vowing to veto the bill if it reached him.

The logic of that argument has always been a little hard to follow, since the measure the Senate is likely to take up, sponsored by Democrat Robert Menendez (N.J.) and Republican Mark Kirk (Ill.), would mandate new sanctions only if Iran failed to accept an agreement by the June 30 deadline established in the ongoing talks. Common sense suggests the certain prospect of more punishment for an already-damaged economy would make the regime of Ali Khamenei more rather than less likely to offer the concessions necessary for a deal.

We gave Mr. Obama’s argument the benefit of the doubt when Congress first considered the legislation more than a year ago. But the president’s logic has been undercut by the manifest willingness of the Iranians to adopt their own pressure tactics — including steps that are considerably more noxious than the threat of future sanctions. On the day before talks resumed between Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif last Wednesday, Tehran announced that construction has begun on two new nuclear reactors. The next day its news agency reported that the case of Washington Post correspondent Jason Rezaian, who has been imprisoned since July 22, had been referred to the Revolutionary Court for “processing.”

The State Department was quick to explain that Iran is not barred by United Nations resolutions or an interim nuclear agreement from building new reactors. Yet by announcing the construction, the regime is making clear its intention to continue expanding, rather than dismantling, its nuclear infrastructure. It’s also demonstrating that it’s not constrained from taking provocative steps during the course of the negotiations — even as the Obama administration argues that countervailing pressure would somehow be a deal breaker.

The case of Mr. Rezaian is particularly disturbing, as he and his family have been subjected to prolonged and gratuitous suffering that violates humanitarian norms and Iran’s own laws. As of Friday, the 38-year-old journalist, who was born and raised in California, had been held for 178 days, longer than any Western journalist arrested by the regime. He has yet to learn the charges against him or be allowed to consult with his lawyer. According to his mother, who was allowed to meet with him last month, he has lost more than 40 pounds.

Charges were brought against Mr. Rezaian at a Dec. 6 court hearing, but he was not able to learn the exact allegations against him. The court to which his case has now been transferred handles sensitive national security cases. Post Executive Editor Martin Baron expressed the hope that the referral will prove to be a step forward that will allow the charges to be deemed baseless. But the fact remains that Mr. Rezaian, a dedicated professional whose reporting was praised by Mr. Zarif, should never have been arrested. It’s difficult to avoid the conclusion that he is being used as a human pawn in the regime’s attempt to gain leverage in the negotiations.

If tactics such as that do not ruin the chance of an agreement, then neither should action by Congress.