[caption id="attachment_153904" align="alignright" width="188"] Israel's Prime Minister Benjamin Netanyahu addresses the American Israel Public Affairs Committee (AIPAC) policy conference in Washington, March 2, 2015.
CREDIT: REUTERS/JONATHAN ERNST[/caption]

(Reuters) - Israeli Prime Minister Benjamin Netanyahu will escalate his campaign against U.S.

President Barack Obama's Iran diplomacy on Tuesday in a speech to Congress whose staging has put unprecedented stress on the two leaders' already strained ties.

Although cold-shouldered by the administration, Netanyahu has offered an olive branch, saying he meant no disrespect to Obama by accepting an invitation to address U.S. lawmakers that was orchestrated by the president's rival Republicans.

As many as one-fifth of Democratic members plan to sit out the speech to protest what they see as a politicization of Israeli security, an issue on which Congress usually unites.

The White House is wary of Netanyahu using the forum to lay bare the closed-door negotiations designed to curb Iran's nuclear drive.

Obama on Monday appeared to wave off any prospect the bedrock U.S. alliance with Israelmight be ruined by the rancor.

"I don’t think it's permanently destructive," Obama told Reuters in an interview. "I think that it is a distraction from what should be our focus. And our focus should be, ‘How do we stop Iran from getting a nuclear weapon?’"

Netanyahu, who has played up his security credentials ahead of a closely contested March 17 election in Israel, has denied his speech would have any design other than national survival.

"I plan to speak about an Iranian regime that is threatening to destroy Israel," he told the pro-Israel U.S. lobby AIPAC on Monday. "The last thing that I would want is for Israel to become a partisan issue, and I regret that some people have misperceived my visit here this week as doing that."

He did not elaborate on the speech's content. One aide said Netanyahu would inform U.S. lawmakers of details on the nuclear talks in hope they would question the administration and thus defer a March-end target date for a framework deal with Iran.

Israel believed members of Congress "do not necessarily know the details of the deal coming together, which we do not see as a good deal," the aide told reporters on condition of anonymity.

Netanyahu wants the Iranians stripped of nuclear projects that might be used to get a bomb - something Tehran insists it does not want. Washington deems the Israeli demand unrealistic.

Under a 2013 interim deal, the United States and five other powers agreed in principle to letIran maintain limited uranium enrichment technologies. U.S. national security adviser Susan Rice argued on Monday that this commitment could not be undone.

"As desirable as that would be it is neither realistic nor achievable," she told AIPAC, over cheers by some audience members supporting Netanyahu's hard-line tack on the Iran talks.

"If that is out goal, our partners will abandon us and undermine the very sanctions we have imposed so effectively together," Rice added, urging U.S. lawmakers not to intervene.

Congress, she said, "shouldn't play the spoiler now."

A deal with Iran is far from guaranteed, given U.S. assessments that more than a decade of carrot-and-stick diplomacy with Iran might again fail to clinch a final accord.

"I would say that it's probably still more likely than not that Iran doesn't get to yes," Obama told Reuters. "It is more likely that we could get a deal now than perhaps three or five months ago. But there are still some big gaps that have to be filled."

By Reuters