Israeli Prime Minister Benjamin Netanyahu’s scheduled appearance before the U.S. Congress next Tuesday will be the “speech of his life,” an Israeli official in Washington told Al-Jazeera.

Rejecting pleas from many of Israel’s most reliable supporters in the United States to avoid playing politics with a vital strategic relationship, the Israeli prime minister is expected to strike a Churchillian pose. He’ll brand an impending nuclear agreement between world powers and Iran as another Munich 1938 — appeasement of an aggressive regime, which Netanyahu insists is determined to make nuclear weapons in order to destroy Israel.

That may be a familiar message from the Israeli leader, but it’s likely to be overshadowed by the poor political optics of the event. Vice President Joseph Biden will be traveling in Central America, which means he won’t take up the seat behind Netanyahu and next to House Speaker John Boehner that the vice president traditionally fills at a joint meeting of both chambers of the legislature. More than two dozen other Democrats, including several senators, are also boycotting the speech in a serious disruption of the bipartisan support Israel traditionally enjoys on Capitol Hill.

President Barack Obama — who was not consulted about Netanyahu’s visit — will not meet the Israeli leader in Washington, citing a desire to avoid influencing Israel’s imminent election. National Security Adviser Susan Rice on Tuesday called Netanyahu’s decision to accept Boehner’s invitation “destructive” to the U.S.-Israel relationship. Secretary of State John Kerry questioned Netanyahu’s strategic judgment on Iran by pointing out in the Senate on Tuesday that the Israeli leader — then a private citizen — had also urged the U.S. to invade Iraq in 2002.

Netanyahu seems to believe that he can still derail what he calls a “bad deal” with Iran, even as he concedes that he may be too late to stop one. Speaking at a campaign rally in Israel on Wednesday in pursuit of an unprecedented fourth term as prime minister, Netanyahu denounced the United States and its allies for having “given up” on their commitment to prevent Iran from becoming a nuclear weapons state. "From the agreement that is forming it appears that they [world powers] … are accepting that Iran will gradually, within a few years, develop capabilities to produce material for many nuclear weapons," he said. "They might accept this but I am not willing to accept this."

The Obama administration vehemently rejects Netanyahu’s charge, insisting that its diplomacy of the past 18 months is the best, and perhaps only, way to prevent Iran from acquiring nuclear weapons capability.

Given that the administration has already written off Netanyahu’s objections and is no longer even sharing details of the talks with his government, it remains unclear what the Israeli leader hopes to accomplish in Washington beyond playing to his domestic political base. If anything, the fact that Israel is so vocally denouncing the proposed deal makes it easier for Iranian hardliners to accept it.

Netanyahu’s planned speech also appears to have undermined Israel’s influence on Capitol Hill. By forcing Democrats to choose between the Israeli leader and Obama, Netanyahu is weakening congressional opposition to a deal and support for new sanctions against Iran — the very opposite of the outcome he intended.

Away from the drama in Washington, a nuclear agreement appears to be taking shape in high-level U.S.-Iran negotiations in Europe. The accord is expected to last 15 years, with some restrictions on Iran’s enrichment capacity phased out after a decade. Key elements would remain in effect in perpetuity, namely Iran’s compliance with the nuclear Non-Proliferation Treaty (NPT) which permits the peaceful use of nuclear energy but enjoins member states — apart from the U.S., Britain, France, Russia and China — from possessing nuclear weapons.

According to individuals familiar with the content of the negotiations, a key breakthrough occurred several weeks ago — awkwardly, in Munich — in talks between Kerry and Iranian Foreign Secretary Javad Zarif. Iran provisionally agreed to mothball centrifuges not currently in operation, and to limit the enrichment capacity of those still churning out low enriched uranium (LEU). Iran has also agreed to send out most of its stockpile of LEU, probably to Russia, for possible conversion into fuel for Iran’s only functioning nuclear energy reactor at Bushehr. This would prevent Iran from amassing a stockpile of enriched uranium sufficient for further enrichment to create the core of a nuclear weapon.

Taken together, these measures would extend to one year the “breakout period” — the time required for Iran to enrich a single bomb’s worth of uranium to weapons grade. Any country with a full fuel cycle civilian nuclear program has the capacity to create weapons-grade uranium if it quits the NPT with its attendant inspection regimes that are supposed to account for every ounce of nuclear material. The goal set by the Obama administration and world powers for an acceptable compromise was one with sufficient safeguards to ensure that it would take Tehran at least a year to produce a single bomb’s worth of enriched uranium should it decide to leave the NPT.

There are a number of ways to achieve a one-year breakout. David Albright, founder and president of the Institute for Science and International Security and a frequent witness on Capitol Hill, said this week that it could be obtained with Iran retaining about 6,000 of its 19,000 rudimentary centrifuges, known as IR-1s, and a stockpile of 500 kilograms of LEU. Albright said he would need to see the rest of the agreement before he could publicly support it, but his calculations on centrifuge numbers and stockpiles appear to be converging with that of the Obama administration and could help sell a deal to Congressional and other skeptics.

Meanwhile, the controversy over Netanyahu’s speech keeps mounting as prominent Democrats such as Virginia’s Sen. Tim Kaine vowed to boycott the event. Even some Republicans are uneasy. Dov Zakheim, a former under secretary of defense in the George W. Bush administration, wrote Wednesday that Netanyahu — who rejected an overture to meet separately with Senate Democrats — was jeopardizing not just his chances of rallying opposition to an Iran nuclear accord, but also of maintaining U.S. opposition to Palestinian statehood moves at the U.N. and even future military aid to Israel.

Netanyahu “is right to oppose a deal that he views as bad for his country,” wrote Zakheim. “But he is wrong to put the Israeli-American relationship at risk.”