WASHINGTON/JERUSALEM (Reuters) - Signs are growing that Israeli Prime Minister Benjamin Netanyahu's planned speech to Congress against a possible nuclear deal with Iran could damage his country's broad alliance with the United States.

The right-wing leader's acceptance of a Republican invitation to address the U.S. legislature already brought Netanyahu's long-strained relations with President Barack Obama to a new low due to the overture's partisan nature.

And while U.S. and Israeli officials insist that key areas of cooperation from counter-terrorism to intelligence to cyber security will remain unaffected, the deepening divide over the Iran talks is shaping up as the worst in decades.

Israel fears that Obama's Iran diplomacy, with an end-of-March deadline for a framework nuclear agreement, will allow its arch foe to develop an atom bomb. Tehran denies it is seeking nuclear weapons.

Previously Israel has always been careful to navigate between the Republican and Democrat camps. The planned address, however, has driven a rare wedge between Netanyahu's government and some congressional Democrats. Some two dozen or more of them plan to boycott the speech, according to unofficial estimates.

Embarking on Sunday on what he described as "a fateful, even historic mission", Netanyahu, who is running for re-election in a March 17 ballot, has framed his visit as being above partisan politics or to win votes.

"I feel that I am an emissary of all Israel's citizens, even those who do not agree with me, and of the entire Jewish people," Netanyahu said as he boarded his plane in Tel Aviv.

Hard-line U.S. supporters of Israel insist Netanyahu should take center-stage in Washington on Tuesday to sound the alarm over the possible deal, even at the risk of offending the White House and long-time Democratic supporters.

But a U.S. official, speaking on condition of anonymity, said the "politicized" nature of Netanyahu's visit threatens "what undergirds the strength of the relationship".

As one former U.S. official put it: "Sure, when Netanyahu calls the White House, Obama will answer. But how fast will he be about responding (to a crisis)?"

At the same time, many people on both sides of the debate over Netanyahu's speech point to the two countries' history of being able to "compartmentalize" diplomatic disputes to preserve cooperation on other shared priorities.

FRAYING RELATIONSHIP

Last month, U.S. officials accused the Israeli government of leaking information to the Israeli media to undermine the Iran negotiations and said it would limit further sharing of sensitive details about the talks.

And Israelis have long fretted over the possibility that Washington might not be as diligent as before about shielding Israel at the United Nations and other international organizations.

One Israeli official said this was becoming more worrisome as the Palestinians are resorting increasingly to global forums like the International Criminal Court to press grievances against Israel and as Europe is losing patience with Israel over settlement building on occupied land.

Netanyahu is expected to use his speech to urge Congress to approve new sanctions against Iran despite Obama's insistence that he would veto such legislation because it would sabotage nuclear talks.

Using strong language, Obama's national security adviser Susan Rice called the political partisanship caused by Netanyahu's coming address "destructive to the fabric of the relationship" with Israel.

"What the prime minister is doing here is simply so egregious that it has a more lasting impact on that fundamental underlying relationship," said Jeremy Ben-Ami, head of J Street, a liberal pro-Israel lobbying group aligned with Obama’s Iran policy.

Netanyahu, who will address the influential pro-Israel lobby AIPAC on Monday, has remained defiant. Even so, he is expected to try to keep tensions from spiraling.

(Additional reporting By Patricia Zengerle and Mark Hosenball in Washington and Ori Lewis in Jerusalem; Editing by Jeffrey Heller and Raissa Kasolowsky)