Just as US diplomats begin final-stage talks with Iran over its nuclear programme, Obama is stepping up his intense lobbying within his own party to prevent Congress from passing new Iran sanctions legislation.

At present, 16 Democrats are among the 59 senators who have sponsored a bill that would impose swingeing new restrictions on Iranian oil exports if Tehran violates the interim nuclear agreement that was finalised at the weekend. Iran has threatened to pull out of the talks if the bill is passed.

Coming during one of the most difficult periods of his presidency, Obama’s efforts to pressure Senate Democrats could become a defining moment in his often troubled relationship with Congress. Although Obama has threatened to veto the legislation, 67 votes in the Senate would overcome a veto. Support in the House of Representatives is also expected to be veto-proof.

At the same time, the sanctions bill is putting many Democrats in a delicate position at the start of an election year. The legislation is being pushed strongly by pro-Israel lobby groups, such as the American Israel Public Affairs Committee (AIPAC), which are influential among some party donors. Yet opinion polls show that Obama’s diplomacy towards Iran is very popular among voters, especially Democrats.

While White House officials have warned that the sanctions bill amounts to a “march to war”, Obama adopted a more restrained tone. “Now is the time for us to allow the diplomats and technical experts to do their work,” he said on Monday afternoon.

US officials argue that passing more sanctions will expose the new Iranian government to criticism from its own hardliners and will make it harder for Iran to make the substantial concessions that will be required in any final-stage nuclear deal.

Iran’s hardliners seem curbed by the country’s supreme leader and ultimate decision maker Ayatollah Ali Khamenei not to oppose the nuclear agreement.

However, they are waiting for the first chance to attack President Hassan Rouhani’s nuclear diplomacy and to undermine his government before the competition formally begins later this year for parliamentary elections early next year, said one reform-minded analyst.

Mohammad Hassan Ghadiri Abyaneh, a hardline analyst, said any new US sanctions meant “those who believe diplomacy and negotiations and smiles could help ease US atrocities are wrong”.

Even though support for the sanctions bill has grown in recent weeks, there are no signs yet that Senate majority leader Harry Reid will put it up for a vote. Moreover, some of the Democratic senators who have sponsored the bill have also shown signs of flexibility.

“I want to talk to some of my colleagues,” Richard Blumenthal, a Connecticut Democrat, said on Monday. “I’m encouraged and heartened by the apparent progress [in diplomacy with Iran] and certainly the last thing I want to do is impede that progress.”

One Senate aide said the senior Democrats supporting the bill might back off in the short term, but would renew their push if there were any signs that Iran was backtracking on the interim agreement or that its economy was recovering strongly.

In one incident with possible political repercussions, federal prosecutors in Connecticut arrested a man of Iranian origin last week on charges that he was attempting to send confidential information about the F-35 fighter jet programme to Iran.

Mozaffar Khazaee, who became a US citizen in 1991 and who used to work for defence contractor Pratt & Whitney, was arrested as he attempted to board a flight that would have taken him to Iran, prosecutors said.

The Republican senator most closely associated with the new sanctions bill said it would be a mistake to delay approval. “The American people will have committed a grievous foreign precedent similar to Neville Chamberlain giving away Czechoslovakia at the beginning of second world war,” said Mark Kirk, an Illinois senator, Huffington Post reported.