Yesterday, the pro-Israel lobby AIPAC released a letter signed by 88 of 100 U.S. senators calling on President Obama to oppose one-sided U.N. intervention in the Israeli-Palestinian conflict. “At this delicate stage the international community should both provide hope to the parties and avoid taking action that would harm the prospects for a meaningful progress,” it read. “Even well-intentioned initiatives at the United Nations risk locking the parties into positions that will make it more difficult to return to the negotiation table and make the compromises necessary for peace.”Signatories to the letter included everyone from progressive stalwarts like Elizabeth Warren and Chris Murphy, to Hillary Clinton running mate Tim Kaine, to conservative leaders like John McCain and Mike Lee. It’s an impressive bipartisan display seldom seen in today’s polarized Washington, and a reflection of historically high pro-Israel sentiment in the United States.That said, the letter is largely symbolic. Obama and past presidents can and have disregarded Congress on Israel and many other matters of foreign policy. Moreover, with its emphasis on “one-sided” U.N. resolutions, the missive leaves room for U.S. peace parameters, which would be presented as a balanced proposal.But precisely because the letter is a symbolic gesture, it is particularly noteworthy which senators did not sign it—chief among them Republican leading lights Ted Cruz, Marco Rubio and Nebraska’s Ben Sasse. All three legislators are seen as the future of the party, and all three consider themselves passionately pro-Israel. The reasons they did not sign, then, offer a window into the shifting pro-Israel landscape in DC and the challenges any effort to forge a bipartisan consensus will face in the years to come.For Cruz, the letter’s enthusiastic embrace of the two-state solution gave him pause. “I support the spirit of Sens. Gillibrand’s and Rounds’ letter to President Obama, which is to urge him to oppose any anti-Israel activities at the United Nations Security Council,” the Texas senator said in a statement. “Unfortunately, the language in the opening paragraph declaring the ‘two-state solution’ as the ‘only’ resolution to the conflict between Israel and the Palestinians undermines this well-intentioned effort, and makes it impossible for me to sign. This matter is an internal one for Israel to decide, and it is not the place of the United States—or the United Nations—to impose a solution on a sovereign nation.”Cruz’s stance reflects an important shift on the right of the Republican party. George W. Bush was the first American president to make the two-state solution the official policy of the U.S. government. That goal has since been adopted by both major political parties in America and Israel, and formed the backbone of the bipartisan approach to the Israeli-Palestinian conflict. In recent years, however, support for two states has eroded on the right, in part due to skepticism among Republican voters and in part due to Prime Minister Netanyahu’s hedging on it during his reelection campaign. The two-state solution was subsequently removed from the 2016 Republican party platform.By contrast, Florida Senator Marco Rubio did not overtly distance himself from the letter’s two-state solution rhetoric, but argued that Israel lacked a Palestinian partner for peace at this time, and that the letter was insufficiently forceful on this point. “The fact that U.S. Senators must plead with an American president to not abandon Israel at the United Nations is a disturbing sign of how much the Obama Administration has undermined our alliance with Israel,” Rubio said. “I join my colleagues in hoping President Obama does not break from our country’s longstanding tradition of supporting Israel at the U.N., but I reject any notion that Israel is at fault in the current impasse with the Palestinians. Israel is America’s closest ally in the Middle East and has made good faith efforts to pursue peace. However, Israel does not currently have a viable partner for peace. Palestinian leaders continue to incite violence against innocent Israelis even as they seek to elevate their cause at the U.N. It is Palestinian leaders, not Israeli leaders, who stand in the way of the peace all Israelis and Palestinians deserve.”Both Rubio and Cruz have close ties with GOP megadonor Sheldon Adelson, who has been funding a right-wing competitor to the bipartisan AIPAC, the Israeli American Council. Whether their absence from AIPAC’s latest initiative reflects a turn towards other more hawkish pro-Israel outlets will be worth watching.Finally, Nebraska senator Ben Sasse, a Yale PhD and former university president who has emerged as one of the right’s leading anti-Trump voices in Congress, also did not sign out of concern that the letter did not draw a sufficient contrast between Israel and the Palestinians. “While I share my colleagues’ aim to stand with Israel,” he said, “I am concerned that the United Nations treats Israelis and Palestinians as equivalent partners—there is simply no comparison. Israel has worked toward peace but Palestinian leaders still incite violence. Israel is committed to freedom and Hamas uses civilians as human shields. Too often the United Nations pretends that there is some kind of moral equivalence between these two parties and, until that stops, the U.N. will not broker any solution for our ally Israel that Americans can support.”Taken together, these dissents from the younger wing of the Republican party reflect the increasing difficulty of crafting a pro-Israel line that can attract broad bipartisan support. For now, the erosion is only on the margins, but with J Street pulling politicians on the left and Adelson’s IAC making a play for the right, it remains to be seen if the center will hold.UPDATE: The Israeli American Council provided the following response to this piece:The IAC is a bipartisan organization, with members, funders, and leaders who hold views that span across America’s political spectrum.\n\n\n\nThe IAC is not in competition with AIPAC. On the contrary, it works closely with AIPAC in many areas to advance the two organization’s shared goal of strengthening the U.S.-Israel relationship. While advocacy is a core part of the IAC’s mission, it is only a part of the organization’s work. The IAC supports a range of social, cultural, and educational programs to strengthen the Israeli-American community and the broader Jewish-American community as well.\n\n\n\nFinally, any implication that pressure from the IAC in any way is what drove these three Republican Senators not to sign on to this letter is just plain false. In fact, the IAC supports this letter and has commended those elected officials who signed it. We have joined these legislators and other pro-Israel groups in strongly urging the Obama Administration to veto any resolution in the UN Security Council seeking to impose terms on Israel.