Remember Sen. Tom Cotton’s letter to Iran? These days, you can bet that the leaders of Iran do.

The letter, sent in March 2015, was one of those events that seemed like a major outrage at the time but now feels like such ancient history that the letter might as well have been written on parchment. At the height of the debate over the Obama administration’s nuclear diplomacy with Iran, the Arkansas senator organized a group of 47 Republican colleagues to send a letter to the “leaders of the Islamic Republic of Iran” explaining “features of our Constitution … which you should consider seriously as negotiations progress.” Noting that any deal would be merely an “executive agreement between President Obama and Ayatollah Khamenei” rather than a formal treaty ratified by Congress and that Barack Obama would be leaving office in January 2017, the letter warned that the next president would revoke it “with the stroke of a pen.”

Democrats were livid about what they saw as a Republican attempt to sabotage ongoing diplomatic negotiations. It certainly was that, but the thing is, Cotton was completely right: On May 8, 2018, Donald Trump pulled the U.S. out of the deal the Obama administration had negotiated and re-imposed sanctions lifted as part of it. Tensions between the two countries have been rising ever since.

The five other countries that were party to the deal and the European Union are still committed to it, as is Iran, but just barely. This week, Iranian nuclear officials announced that they had quadrupled the country’s uranium enrichment production, putting them on a path to exceed limits set by the accord.

If the Iranian regime stays in compliance with the deal, it may be only because they know Trump won’t be in office forever, either. Most of the Democratic candidates running for president in 2020 have promised to rejoin the Iran deal, which would entail lifting the sanctions that Trump has re-imposed. Ironically, Democrats have been borrowing from the Republicans’ old playbook with former Secretary of State John Kerry, who spent years negotiating the original deal, meeting with his Iranian counterparts in an attempt so salvage it. In both cases, critics of these actions invoked the Logan Act, the oft-cited but never enforced law barring U.S. citizens from conducting unauthorized negotiations with foreign governments. “He told them to wait out the Trump Administration!” an enraged Trump tweeted.

Waiting out the Trump administration is a popular idea these days. Susan Thornton, the former acting assistant secretary of state, told a gathering in Shanghai earlier this month that China should be patient amid the ongoing trade war and not rush to resolve it, telling the audience, “If this skeptical attitude towards talking diplomacy continues in this administration, you might have to wait till another administration.” The parties to the Paris climate agreement may also be somewhat heartened that every major Democratic candidate has pledged to keep the U.S. in the agreement. (The U.S. will technically still be a party to the Paris agreement until November 2020 at the earliest, despite Trump’s withdrawal announcement in 2017.)

This isn’t a very encouraging state of affairs, and not only because there’s a decent chance Trump will be reelected in 2020. If it becomes the norm that agreements struck by a U.S. president will be kept only so long as that president’s party is in power, why would any country ever sign a deal with the U.S. ever again? If the entire disposition of U.S. foreign policy transforms depending on whether a Democrat or a Republican occupies the White House, it will have a destabilizing impact on international relations long after Trump leaves office.

Politics has never entirely stopped “at the water’s edge.” Richard Nixon’s campaign scuttled Vietnam peace talks by promising to change the Lyndon Johnson administration’s policies. The Carter-to-Reagan transition was no doubt almost as whiplash-inducing for foreign governments as the Obama-to-Trump transition. But presidents of both parties have, at the very least, shared a disposition toward international alliances and broad policy objectives that contrasts with domestic political divides. This hasn’t always been for the best—the bipartisan foreign policy consensus has gotten the U.S. into a number of wars it should never have fought—but it has allowed for a certain level of continuity that makes diplomacy possible.

The exceptions to the rule are those occasions when the culture war bleeds into foreign policy. One example is the Mexico City Policy, known by opponents as the Global Gag Rule, a draconian rule introduced by the Reagan administration requiring NGOs that receive U.S. funding to certify they will not perform abortions or promote abortion as a method of family planning. The policy was rescinded under Bill Clinton, reintroduced under George W. Bush, rescinded under Obama, then reintroduced and made even more restrictive under Trump. The policy is bad enough on its own, restricting needed aid to many organizations providing vital family planning and HIV/AIDS services. The fact that these organizations don’t even know what the U.S. funding policies will be from year to year, and can’t plan their activities accordingly, makes the problem worse.

In today’s U.S. political climate, all issues are becoming culture war issues, and more and more official U.S. policies abroad are coming to resemble the Mexico City Policy—switches to be flipped on or off depending on which domestic constituency the party in power wants to appeal to.

There traditionally has been less daylight between the parties on foreign policy, and it comes up much less than domestic issues in presidential campaigns, but ironically, partisanship may come to have more of an impact on foreign policy: In an era of congressional gridlock, it’s a rare area where the president can act relatively unfettered. Even if Bernie Sanders were to become president, it’s unlikely he’d be able to get a controversial domestic policy like Medicare for All passed by Congress. But he would definitely be able to pursue a vastly different policy on Iran.

In today’s U.S. political climate, all issues are becoming culture war issues.

This dynamic has only been getting more acute in recent years. The near impossibility of getting Republican support meant that Obama had to carry out most of his signature diplomatic achievements—the Iran deal, the Paris accord, the normalization of relations with Cuba—via executive action. That also meant those achievements could be undone by executive action.

Treaties, as defined by Article II of the Constitution, require a two-thirds majority of the Senate for ratification, so they have become almost unheard of. George W. Bush submitted more than 100 treaties to the Senate for ratification during his time in office, roughly on par with his 20th-century predecessors. Obama submitted just 38, only 15 of which were ratified. Trump has submitted just two so far: an amendment to an earlier fisheries agreement and an update to the North Atlantic Treaty allowing North Macedonia to join NATO. (Trump wasn’t too enthusiastic about this.)

Meanwhile, Trump has pulled the U.S. out of negotiations for the planned Trans-Pacific Partnership and the 1987 Intermediate-Range Nuclear Forces treaty with Russia. Even the Universal Postal Union hasn’t been safe.

Nearly all the Democratic presidential candidates, to the extent they discuss foreign policy at all, emphasize the importance of rebuilding international alliances. If any of them becomes president, he or she would probably work to rejoin many of the deals and institutions that Trump has pulled out of and reaffirm the U.S. commitment to those, notably NATO, that Trump has simply trashed. But it will be immeasurably more difficult for the next Democratic president to replicate even the limited diplomatic achievements of the Obama years. Why would the leader of any U.S. adversary want to repeat the experience of Iran or Cuba by spending political capital on a rapprochement with the U.S., only to see the offer humiliatingly rescinded by the next president?

The problem isn’t just treaties and agreements. The way Americans view the world, and other countries, is increasingly polarized. Polls show increasing divergences between the parties in views on Mexico, Iran, Russia, Germany, Canada, and other countries. Iran may not be particularly popular on either side of the aisle, but prominent voices on Democratic foreign policy, including Sanders’ foreign policy adviser, have been arguing that Saudi Arabia is viewed as worse. Whether you think the Iranians or the Saudis are a bigger threat to human rights and regional stability may soon become an indicator of partisan loyalty.

On the flip side, while Americans are broadly supportive of Israel, there is an increasingly stark partisan divide in views of the Israeli government and the Israeli-Palestinian conflict. This shouldn’t be all that surprising considering that few leaders have inserted themselves into America’s partisan warfare as consciously or effectively as Israeli Prime Minister Benjamin Netanyahu. While he has declared that “Israel should always remain a bipartisan issue,” Netanyahu has effectively aligned himself with Republicans, at least since the debate over the Iran deal. Mideast peace, too, is a culture war issue now.

The downside of this is it has become politically easier for Democrats to criticize Israeli policies—some in quite stark terms—but it’s hard to say the strategy hasn’t paid off for Netanyahu or for Trump. The U.S. administration has moved forward on a wish list of Israeli policy demands such as scuttling the Iran deal, moving the U.S. Embassy to Jerusalem, and recognizing Israeli sovereignty over the Golan Heights. The effect all this will have on stability in the Middle East, Palestinian human rights, or even Israel’s own long-term interests are another story.

Other leaders may follow Netanyahu’s lead by attempting to align themselves with a U.S. political party rather than the U.S. as a whole. Just look at Hungary’s controversial right-wing Prime Minister Viktor Oban, who was the first world leader to endorse Trump’s candidacy in 2016 and was subsequently feted by Trump in the Oval Office last week ahead of crucial European Parliament elections, despite both Bush and Obama previously giving him the cold shoulder.

Countries may never be entirely neutral topics in each other’s partisan politics—the U.S. certainly hasn’t always been. But at least the ideal state of affairs is that they’re willing to conduct business with whatever party is in power. And that only works if there’s at least a certain level of continuity in foreign policy outlook and strategic goals as well as a baseline willingness to honor agreements signed by the previous administration, even if the new one doesn’t like them very much. If current trends continue, America’s partisan divide will continue to widen, and foreign policy questions will increasingly be treated as culture war fights between the parties.

If future presidents share Trump’s disdain for international institutions and agreements, the credibility of U.S. foreign policy stances will suffer. And if these partisan swings become predicable—in other words, if countries know in advance that certain U.S. policies and agreements only apply when one party or another is in office—it will increase incentives for foreign powers to try to influence and meddle in U.S. elections. What’s a few cyberattacks or Facebook ad buys when the entire foreign policy outlook of the most powerful country is at stake?

Americans are increasingly living in two different countries at home. Soon we may be treated as two different countries abroad as well.