Throughout the Gaza conflict, President Joe Biden has been about as supportive of Israel as its leaders could have hoped. He has issued statements supporting its “right to self-defense,” blocked UN Security Council resolutions calling for a ceasefire, and even chose to move forward with a previously approved US arms sale to Israel worth $735 million.

In short, it seems like the US-Israel alliance is as strong as ever. But beneath the surface, there are signs that the relationship isn’t what it once was. Despite Biden’s firm stance, the US and Israel may be heading for a divorce in the long run.

The most visible of these signs is the rising discontent with Israel among progressive Democrats in Congress. The left flank of the party, represented by Vermont Sen. Bernie Sanders and New York Rep. Alexandria Ocasio-Cortez, has been unsparingly critical of Israel — with Ocasio-Cortez and her allies in the House, like Michigan Rep. Rashida Tlaib (who is of Palestinian descent), calling Israel an “apartheid state.”

But even some more centrist Democrats with strong pro-Israel bona fides are taking a harder stance. Rep. Greg Meeks (NY), chair of the House Foreign Affairs Committee, initially called for a pause on the new arms sale; Sen. Robert Menendez (NJ), chair of the Senate Foreign Relations Committee, has suggested Israel is not taking enough care to avoid killing civilians in Gaza.

“If you don’t follow congressional statements on this stuff over time, it is really hard to explain how remarkable this is,” writes Yousef Munayyer, a nonresident fellow at the Arab Center think tank in Washington, DC.

The Democratic position on Gaza reflects a long-running process of partisan polarization of the Israeli-Palestinian conflict.

The US-Israel alliance, which has its origins in Cold War geopolitics, has been a bipartisan endeavor for decades. But a series of factors — including the actions of specific leaders like Israeli Prime Minister Benjamin Netanyahu and deeper political trends in both countries — has created a partisan imbalance. Republicans have become more pro-Israel than ever, while Democrats are becoming more and more sympathetic to the Palestinian cause.

“People are becoming increasingly aware of the indefensible human rights situation on the ground in Gaza,” Rep. Ilhan Omar (D-MN) told me via email. “This includes a diverse coalition of progressive Jewish-Americans, Muslim-Americans and others who see our humanity as directly tied to one another.”

Historically, the cornerstone of the US-Israel alliance has been bipartisan support — both on Capitol Hill and among the American public. You need both parties to continue approving US aid to Israel in Congress; you need policy continuity in the White House to ensure unchanging US support in international forums like the UN. The more partisan Israel becomes, the weaker the alliance gets.

Unless something fundamental changes, it’s easy to see how the US-Israel alliance could continue to unravel in the long term. Biden may be the last Democratic president to give Israel a blank check during a war.