Bernie Sanders did something previously unheard of in last night's CNN debate: He stood up for Palestinians' humanity.

"As somebody who is 100 percent pro-Israel, in the long run," Sanders said, "we are going to have to treat the Palestinian people with respect and dignity."

For many, this seems banal: Sanders is asserting that the US needs to understand Israeli and Palestinian beliefs to ultimately resolve the conflict.

But in the context of a presidential primary, it's historic. For years, mainstream candidates have hewed to a narrow "pro-Israel" line, blaming the conflict on the Palestinians and pledging strong support for Israeli policy. Democrats and Republicans have done this in different ways, to be sure, but generally speaking the Israeli perspective dominates.

Yet last night, the most successful Jewish candidate in primary history flipped the script. He inserted a line of argument, long common on the Israeli left, into the American political mainstream.

The ensuing argument he had with Clinton is the conversation Democrats (and perhaps America) have needed on Israel for years.

The Clinton-Sanders exchange is an internal liberal argument going public

Sanders, as his comments make clear, is not taking a hard-line anti-Israel position. He emphasized his support for Israel, and his time as a young man living in the Jewish state.

Rather, Sanders is championing what's commonly called liberal Zionism, which holds (among other things) that you can both support the existence of Israel and be deeply critical of its treatment of the Palestinians at the same time.

"Of course Israel has a right to defend itself, but long term there will never be peace in that region unless the United States [recognizes] the serious problems that exist among the Palestinian people," Sanders said during the debate.

Clinton's response, where she highlights her time as secretary of state, is extremely telling. She says that she, too, has stood up for Palestinians — but in private negotiations:

I'm the person who held the last three meetings between the president of the Palestinian Authority and the prime minister of Israel. There were only four of us in the room, [Prime Minister] Netanyahu, [President] Abbas, George Mitchell, and me. Three long meetings. And I was absolutely focused on what was fair and right for the Palestinians.

This has, for years, been the standard line among mainstream pro-Israel Democrats. Criticize Israeli mistakes, to be sure, but do it in private. Public distance between the United States and Israel erodes trust between the two sides, which makes it hard for the US to broker peace negotiations.

Many liberal Zionists, like Sanders, find this unsatisfying. They argue that Israel is headed on a self-destructive course, expanding settlements in the West Bank and choking off the possibility for a two-state solution. Backroom arguments aren't good enough anymore; the US needs to warn Israel away from its current path.

This is the meaning of Bernie's sharpest dig in the debate: "There comes a time when if we pursue justice and peace, we are going to have to say that Netanyahu is not right all of the time."

Sanders is saying that the US needs to publicly acknowledge Israeli mistreatment of the Palestinians, because the reality is that Palestinians are indeed suffering terribly — and right-wing Israeli policies, championed by politicians like Netanyahu, are partly to blame. Clinton, he argues, has at various points (such as her recent speech to AIPAC, the pro-Israel lobby) been too unwilling to emphasize.

If Sanders gets his way, this would be a serious revision to the standard Democratic approach to the Israel-Palestine conflict. We've started to see it under Obama — Obama has been publicly critical of Netanyahu on both settlement expansion and his attempts to obstruct the Iran deal. But Sanders wants to extend this criticism, arguing that the United States should take a more active role in championing Palestinian rights.

This isn't an argument over whether the United States should continue to treat Israel as an ally and friend. It's a question of how critical America should be of its ally over its treatment of the Palestinians — one that has long divided American liberals and Jews but is only starting to creep into mainstream Democratic discourse.

Sanders busted a big taboo — and heralded a new Democratic discourse on Israel

Sanders is far from a perfect champion for the liberal Zionist position. Just hours before the debate, he suspended his Jewish outreach coordinator, Simone Zimmerman, for being overly harsh in her criticism of Netanyahu (disclosure: Zimmerman is a personal friend).

But Sanders's debate advocacy matters nonetheless. His unapologetically liberal Zionism is sending a signal to Democrats that it's okay to be more critical of Israel. He's a Jew who used to live in Israel; in recent polls, roughly half of all Democrats nationally have said they want him to be the party's presidential nominee. It's hard to overemphasize how taboo busting Sanders's advocacy of Palestinian rights is.

"Bernie is opening a lot of space for a generation of Democratic politicians, and maybe for Clinton, re Israel," BuzzFeed editor Ben Smith tweeted last night. And he's right.

Now, if Sanders didn't speak for a large contingent on the American left, his positions wouldn't matter. But opinion polls, and deeper structural trends in American politics, suggest that he does.

Look, for instance, at a February Gallup poll on Israel-Palestine. That poll found that a majority of Democrats, like virtually all American demographics, sympathize with Israel over the Palestinians.

But it also found that a majority of Democrats favor creating a Palestinian state: "58% of Democrats supporting the establishment of a Palestinian state, compared with 26% of Republicans," Gallup's Lydia Saad reports.

So while Democrats are quite sympathetic to Israel, they also believe Palestinians deserve a state of their own — unlike Republicans, who tend to sympathize with Israel in much larger numbers and be skeptical of Palestinian statehood. Sanders's liberal Zionism, then, is no longer obviously a losing position for Democratic politicians to take.

This view could only to strengthen over time. Republicans are taking an increasingly hard-line view of being pro-Israel, owing largely to evangelical and neoconservative influences inside the party. Meanwhile, increasingly important Democratic constituencies — younger voters, black voters, Latino voters — all support Israel at lower rates than do other American demographic groups. And these demographic groups are becoming an increasingly important constituency in the Democratic primary.

Republican politicians will have growing political incentives to attack Democrats as insufficiently supportive of Israel. It'll play well to both their base and the median voter. That threatens to code support for Israel, once a bipartisan issue, as a Republican one. The more it coded as partisan, the more comfortable Democratic politicians will become in criticizing Israel.

Sanders' view of Israel, then, is likely to become more mainstream, not less. His performance last night is the beginning of a long process of mainstreaming a more critical view of Israel in the Democratic Party. More traditional Democrats like Clinton are probably going to need to start learning how to deal with it.