An official from the American Israel Public Affairs Committee, the leading pro-Israel lobby in the US, on Thursday blasted Prime Minister Benjamin Netanyahu for harming the opposition to the Iran nuclear deal by insisting on addressing Congress on the issue in March.

“Netanyahu’s speech in Congress made the Iranian issue a partisan one,” the AIPAC official told Israel’s Walla news. “As soon as he insisted on going ahead with this move, which was perceived as a Republican maneuver against the president, we lost a significant part of the Democratic party, without which it was impossible to block the agreement,” said the official, who asked not to be named.

AIPAC’s spokesman Marshall Wittmann dissociated the organization from the remarks. “The comments by the purported ‘AIPAC official’ to Walla News about the prime minister do not represent or reflect the views of our organization and were not authorized by us,” he told The Times of Israel. Ahead of Netanyahu’s speech to Congress, he also noted, AIPAC made plain it firmly supported the prime minister’s address. “AIPAC welcomes the prime minister’s speech to Congress and we believe that this is a very important address,” Wittmann said at the time. “We have been actively encouraging senators and representatives to attend and we have received an overwhelmingly positive response from both sides of the aisle.”

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Dozens of Democratic lawmakers were absent from Netanyahu’s speech — which had been set up without the knowledge of the White House — including President Barack Obama and Vice President Joe Biden.

AIPAC has been at the forefront of the battle against the agreement reached in July between Iran and the world powers, which curbs Iran’s nuclear program in return for the lifting of crippling economic sanctions. But, according to veteran Israeli journalist Ben Caspit, the lobby had opposed Netanyahu’s speech to Congress and urged him to reconsider in anticipation of the strain it would place on US-Israel ties. Netanyahu apparently declined, and the speech went ahead.

According to the AIPAC official quoted by Walla, the lobby was not the only source to raise these concerns with the prime minister.

“A few weeks before Netanyahu’s speech in Congress, Democratic senators implored him to cancel, saying it was inappropriate for the Israeli prime minister to work like this behind the back of the president of the United States,” the official said. “Instead of the speech, they proposed that he come to Washington and meet with them behind closed doors, without the TV cameras, so he could explain his position on the Iranian issue.”

The official added: “A lot of people here think that if Netanyahu had accepted their offer, it would have improved our chances [to foil the deal].”

On Wednesday, Democratic Senator Barbara Mikulski (D-Md.) said that she supported the agreement, granting Obama the backing of the 34 senators he needs to ensure the agreement can beat any attempts in Congress to kill it.

In order to scupper the agreement, the opposition to the deal needed 13 Democratic senators to go against their president, who has consistently championed the agreement and vowed to veto any move by the Republican-dominated Congress to disapprove it. And by speaking to Congress in early March, the AIPAC official said, Netanyahu made the issue a purely partisan one.

“What happened was that 50 Democrats boycotted the speech, and many others were there but filled with anger and bitterness toward Netanyahu, who forced them to choose between Israel and the president,” the official told Walla. “From that moment on, the story of the agreement with Iran became Democrats against Republicans, as with many other political issues. This made Obama’s task of persuasion easier.”