On May 5, the Pew Research Center released a big poll of 2,000 Americans that asked their opinions on a range of foreign policy topics. The research didn't get a lot of attention — there was Donald Trump to cover, after all. But buried in the report was a striking finding about the way Americans see the Israel-Palestine conflict.

Most Americans, Pew found, sympathize with Israel over the Palestinians — a standard finding in these polls. But for the first time in 15 years, the poll found that liberal Democrats sympathize more with the Palestinians than with Israel (40 percent versus 33 percent):

When Pew broke apart the data by presidential candidate preference, it found something similar: While a majority of Hillary Clinton supporters took Israel's side, Bernie Sanders supporters backed the Palestinians by a 39-33 margin.

Now, it's possible that this is just a blip — the polls showed a similar decline in sympathy for Israel in the early 2000s, in the middle of the second intifada. But although there are some pretty clear immediate causes for this recent surge in support for Palestinians, there are good reasons, rooted in American partisan politics, to believe this may actually be part of a longer-term trend.

The immediate causes of the spike: friction between Obama and Netanyahu

The immediate cause of the decline in Israeli popularity among liberal Democrats seems clear: two years of bitter fights between Israeli Prime Minister Benjamin Netanyahu and President Barack Obama.

The two men have never really gotten along. They clashed early in Obama's administration over West Bank settlements, and Netanyahu all but openly campaigned for Mitt Romney in 2012. But the most vicious, public fight was over the nuclear deal with Iran.

Netanyahu campaigned loudly and angrily against the nuclear deal, which he saw as a fundamental threat to Israeli security. He even allied with Republicans to try to rally votes against the deal in early 2015, most infamously by working with Republicans to deliver a speech opposing the deal to a joint session of Congress.

This maneuver in particular infuriated Democrats, who saw it as Netanyahu aligning with the GOP to torpedo one of Obama's signature accomplishments. About 60 congressional Democrats boycotted Netanyahu's speech, a striking number given how popular Israel normally is in Congress regardless of party affiliation.

This public confrontation between Obama and the Democrats on one side and Netanyahu and the Republicans on the other would almost certainly have had an impact on liberal Democrats. A Pew poll in October 2015 found that liberal Democrats were overwhelmingly friendly to the Iran deal, while conservative and moderate Democrats were somewhat hostile. Liberal Democrats are also more likely to be hardcore Obama partisans, and thus would more likely be angry at Netanyahu's perceived slight against the president.

It's likely, then, that Netanyahu's actions spilled over into liberal Democrats' broader perceptions of Israel, making them more sympathetic to the Palestinians in the conflict between the two sides. Indeed, this is something that both Israeli leaders and congressional Democrats warned about at the time.

"Netanyahu’s position will not change the West’s position on the Iranian issue, but his actions bring our relationship with the Americans to an extreme point, and this might extract an unbearable price from us in the future," Meir Dagan, the former head of the Mossad (Israel's CIA), said before the speech. "Talking to my Democratic colleagues, I believe this is not an idle concern," Rep. Jan Schakowsky (D-IL) said in response to Dagan's comments.

The longer-term issue: partisan fights over Israel

Now, it's possible that liberal Democrats are just temporarily skeptical of Israel's position in the conflict, and that they'll eventually get over the Obama-Netanyahu fight and return to their more traditional pro-Israel stance. But there are good reasons, rooted in American partisan politics, to think otherwise.

Republicans and Democrats used to support Israel at roughly similar levels. But in the past several decades, the two sides have diverged, with Democratic support for Israel staying roughly stable and Republican sympathy with the Jewish state climbing dramatically:

According to political scientists, two fundamental forces combined to transform the GOP into the hardcore pro-Israel party we know today. First is the rise of the religious right, which sees hard-line support for Israel as a religious obligation.

"Evangelicals were not politically active until about the 1970s," Elizabeth Oldmixon, a professor at the University of North Texas who studies Israel's role in American politics, told me last year. "Uncompromising support for Israel is something you start to see as evangelicals become more prominent in the party."

Second, the neoconservative movement successfully convinced most Republican leaders that being pro-Israel should be a core conservative value. After Reagan's 1980 election victory, neoconservatism grew in influence, eventually turning into something akin to party doctrine on foreign policy. Neoconservatives saw Israel first as a bulwark against communism, and later as a democratic ally against jihadism, and so pushed hard to elevate support for Israel into a core conservative value.

The result is a swell in Republican support for Israel — of a particularly hard-line sort. What distinguishes Republicans from Democrats on Israel isn't that they call themselves "pro-Israel"; both parties strongly support maintaining the US-Israel alliance. The difference is the character of that support: Democrats are much more open to criticizing Israel on issues like West Bank settlements, whereas Republican support for Israel is more unconditional.

This might seem like great news for Israel, on the face of it. Pro-Israel resolutions still pass Congress by overwhelming bipartisan majorities. It can only be good if Republicans are even more pro-Israel than Democrats, right?

Not necessarily. Republican politicians have growing political incentives to attack Democrats as insufficiently supportive of Israel, as it plays well to both their base, which is increasingly supportive of Israel, and the median voter. But Democrats don't have a similar incentive to attack Republicans as anti-Israel, because their base isn't nearly as committed to the issue. That threatens to code support for Israel, once a bipartisan issue, as a Republican one.

This danger will be especially acute when there is a Democrat in the White House and when that president fights with Israel — as presidents from both parties are bound to do. In such moments, Democratic members of Congress may well feel pressured to choose between supporting their president and the GOP's hard-line vision of what being pro-Israel means. The more this happens, the more Democrats could see support for Israel as a partisan issue and treat it accordingly.

This, of course, is exactly what we saw during the Iran fight. The push for new sanctions on Iran, which would have destroyed the Iran deal, became a Republicans-versus-Democrats issue rather than an Israel-versus-Iran issue — and it failed accordingly. It's possible that this isn't an outlier, but rather a harbinger of what's to come.

A 2015 poll by University of Maryland's Shibley Telhami and Katayoun Kishi asked a variety of different demographic groups about the Israeli-Palestinian conflict. Telhami and Kishi also found that African Americans, Latinos, and younger Democrats are all more sympathetic to Palestinians than the general population. These groups are making up an increasingly bigger part of the Democratic coalition as time goes on, which could accelerate polarization on Israel further.

And Democrats are becoming more liberal over time. The percentage of Democrats identifying as "liberal" jumped from 27 percent in 2000 to 41 percent in 2015, suggesting troubles for Israel if liberal Democrats remain (as has been true for some time) more skeptical of Israel than other Democratic cohorts.

So growing Republican enthusiasm for Israel may be threatening the core foundation of the US-Israel alliance: broad, bipartisan support for the Jewish state. How's that for irony.