The number two Democrat in the Senate, Dick Durbin, said the US Congress would wait for the results of October 1 talks between the Islamic republic and six world powers over its nuclear program.
"I don't think a matter of days will make a difference. And if diplomacy can work, and move us in the right direction, I'd like to give it a chance," said the Illinois lawmaker, who has sharply criticized Iran's nuclear drive.
Some of President Barack Obama's Republican foes have pushed for action now on legislation that targets Iran's reliance on imports to satisfy its people's consumption of gasoline and other refined petroleum products.
Because of a lack of domestic refining capacity, oil-rich Iran is dependent on gasoline imports to meet about 40 percent of domestic consumption.
Iran gets most of its gasoline imports from the Swiss firm Vitol, the Swiss/Dutch firm Trafigura, France's Total, the Swiss firm Glencore and British Petroleum, as well as the Indian company Reliance.
Sanctions targeting firms that do energy business with Iran "would certainly give the president more leverage with Iran," said Republican Senator Saxby Chambliss.
But Democrats and some Republicans have rejected that approach, saying they prefer to wait until after the UN General Assembly, the Group of 20 meeting of major economies, and the October 1 discussions.
"It seems to me that the timetable is the appropriate one. If all of this fails, then we will have to take some actions," said the top Republican on the Senate Foreign Relations Committee, Richard Lugar.
"We ought to approach this one week at a time. This is the week for the United Nations and for the United States to engage with European countries and with others who will be instrumental partners — once again with Russia — to test out where they are," said Lugar.
If negotiations fail, swift congressional action is almost certain amid staggering support for more sanctions: The main versions of the bill have more than 300 backers in the 435-vote House of Representatives and have 75 co-sponsors in the 100-seat Senate.
"Our current efforts to stop Iran from developing nuclear weapons have failed: They're pushing ahead with all speed. They'll continue to do that until we give them some reason not to," said Independent Senator Joe Lieberman.
The lawmaker, a leading backer of the legislation, said he expected the Senate to decide on a course of action after the October 1 talks, because "we'll see then whether there's any hope."
"If I had my druthers, this legislation would have been adopted by the Senate already," Lieberman told reporters. "But I don't have my druthers."
Some lawmakers have expressed frustration at reluctance from some US partners in the talks with Iran, notably Russia and China, to impose stiff economic penalties for Tehran's defiance of global demands it freeze its uranium enrichment program.
"The only way we're going to get a successful sanctions movement is to be realistic and be practical," said Chambliss. "The people of Iran need to understand that we're serious about the way their leadership is acting."
