The two senators have been lining up bipartisan support for new sanctions legislation with a goal of building a majority large enough to overcome an expected veto. Their efforts have been boosted by the administration's seeming willingness to avoid offending Iran or accusing the country of actions that might jeopardize the talks.
"There doesn't seem to be anything they're not willing to tolerate," said Mark Dubowitz, executive director of the Foundation for Defense of Democracies and an expert on Iran sanctions.
Administration officials have defended their conduct, even though they admit remaining differences with Iran could prevent a permanent agreement. They say the interim deal has halted Iran's nuclear progress and even reversed it in some areas, while the sanctions relief, worth $700 million a month, is not enough to heal the country's ailing economy.
"Our strategy has been underestimated from the beginning," Vice President Joe Biden said last month.
Administration officials warn that new sanctions would kill the talks, because Iran would see them as a violation of the interim deal. But they may be key to giving the United States needed leverage to get an airtight agreement that prevents Iran from building a bomb.
The interim deal has taken some of the pressure off Iran's economy, which had spiraled into a deep depression after tough U.S. sanctions were imposed in 2010, Dubowitz said. Nearly two years ago, Iran's economy was on its back, he said. "Now it's on its knees getting up to its feet."
Iran's theocratic rulers also have done a good job raising their people's expectations, creating an atmosphere of optimism that the end of international sanctions is inevitable, the New York Times reported.
That is important, Dubowitz noted, because the United States has largely abandoned attempts to coerce Iran into compliance.
"Without any instrument of coercive statecraft, we're left on the negotiating table with economic sanctions," he said. And though most of them remain in place, U.S. leverage is slipping because Iran is no longer facing economic collapse.
The administration's hard line against Congress on new sanctions has eroded U.S. leverage even further, he said.
"While it might have been politically useful on the Hill, I think it eventually undermined their negotiating position."