The White House is growing more confident it can withstand efforts to frustrate its policy of nuclear talks with Iran, as a furore over the intervention of the Israeli prime minister, Benjamin Netanyahu, appears to be encouraging wavering Democrats to rally around their president.

In the first White House press conference since the Republican House speaker, John Boehner,sparked controversy by inviting Netanyahu to speak before a joint session of Congress amid calls for a tougher approach to Iran, administration officials claimed there was support for their argument that planned legislation authorising new sanctions, if talks fail, would be counter-productive, The Guardian reported.

"I think there is plenty of indication that the at least some members of Congress have found this rather plausible line of argument pretty persuasive," the White House spokesman, Josh Earnest said.

Earnest refused to speculate on a likely vote count, but sympathetic lobbyists in touch with Democratic congressmen claim the polarising impact of Boehner's invitation to Netanyahu is making it harder for Republicans to reach a veto-proof majority for the sanctions bill.

"This move by Netanyahu has definitely backfired in terms of Democrats," said Dylan Williams, director of government affairs at J-Street, a Washington lobby group which describes itself as pro-Israel but supports a two-state peace process for a Palestinian state.

J-Street sent out a letter to its US supporters Friday, urging them to warn their congressmen not to support the visit, which will fall just two weeks before elections in Israel.

"This invitation looks like a thinly veiled attempt to scuttle the critical negotiations taking place right now aimed at ensuring that Iran never acquires a nuclear weapon," said the letter. "Bibi and Obama disagree on how to deal with Iran, and that's fair. But a foreign leader lobbying Congress is inappropriate."

On Friday a senior Israeli opposition figure, Tzipi Livni, accused Netanyahu of leading the country into "crisis and diplomatic isolation", amid growing criticism of his handling of relations with the US.

There is also concern among some Israeli-Americans that the incident may damage broader relations between the two countries.

The national director of the Anti-Defamation League, Abraham Foxman, called on Boehner to rescind the invitation, arguing "I certainly support the sanctions on Iran if the diplomatic deal doesn't come through, but having said that, the invitation and acceptance is ill-advised for either side. It is too important an issue to politicise it."

The White House again dismissed claims that Obama will snub Netanyahu, though the president said Thursday he would not meet the prime minister during the visit. The White House said this was in keeping with a practice that discourages US presidents from meeting with world leaders ahead of elections.