On the surface, it looks like the U.S.-Israel relationship is having its best year ever. In May, President Donald Trump fulfilled Israel’s dream of moving the U.S. embassy to Jerusalem and his administration is preparing a Middle East peace plan that will almost certainly have Prime Minister Benjamin Netanyahu’s blessing. Congress, meanwhile, is poised to approve $3.3 billion in new defense assistance to Israel, a new high.

But there are political undercurrents that spell trouble for what has traditionally been unquestioned U.S. support for Israel, particularly within the Democratic Party on the eve of a midterm election that could swing the balance of power in one or both chambers of Congress and perhaps profoundly and permanently change the dynamic between the longtime allies.

Democratic disillusionment with Israel, which has been quietly simmering for years, gained traction when Netanyahu spoke to a joint session of Congress urging lawmakers to oppose the Iran nuclear deal championed by the Obama administration. Dozens of Democrats defiantly boycotted the March 2015 speech.

The political division became even more apparent when Trump took office two years later and Netanyahu backed some of his most controversial domestic policies. And when Trump embraced cutting off aid to the Palestinians, many Democrats balked at the ongoing humanitarian disaster in Gaza, which the party had previously ignored.

In May, Kentucky Rep. John Yarmuth, the top Democrat on the House Budget Committee, condemned the Israeli government for the “horrific slaughter” of more than 50 Palestinians protesting just inside the border fence of the Gaza Strip.