Throughout Barack Obama's presidency, Republicans in Congress have deployed a strategy that has worked remarkably well for them: oppose, obstruct, and sabotage the Obama administration at every turn.

"The single most important thing we want to achieve is for President Obama to be a one-term president," Republican Sen. Mitch McConnell, then the Senate minority leader, said in 2010.

A few months later, McConnell acknowledged that Republicans had decided to deny President Obama any bipartisan support, not because they necessarily opposed each and every initiative, but to hurt Obama politically. "We worked very hard to keep our fingerprints off of these proposals," he said. "Because we thought — correctly, I think — that the only way the American people would know that a great debate was going on was if the measures were not bipartisan."

This strategy led Republicans to adopt largely unprecedented tactics of obstructionism and sabotage. But no matter how far they went, there was one line they always avoided crossing: undermining US foreign policy.

That line is now being crossed. Republicans, driven by earnest policy disagreements with Obama over his approach to Iran, are bringing the tactics they used to undermine Obama's legislative agenda into the previously sacrosanct realm of foreign policy.

"the GOP are blazing new trails in politicization of foreign policy — and debasement of their institutions"

Republicans are overtly sabotaging not just Obama's Iran policy, but also his constitutionally enshrined authority over foreign policy. This is unprecedented. If the trend continues — Republicans have already extended their efforts to Obama's relationship with Israel — it endangers not just US policy toward the Middle East, but the very way that the United States makes foreign policy.

The possible implications for the United States and its role as global leader should worry Americans of every political stripe.

Republicans are adapting the tactics they used against legislation like Obamacare to Obama's foreign policy

Until now, for all the tactics of obstruction Republicans used against Obama's legislative agenda, they generally treated foreign policy as sacrosanct. They got close only once before, when they threatened to block Obama's 2010 nuclear disarmament treaty with Russia. But they backed down when foreign policy graybeards from Henry Kissinger to Colin Powell told them to knock it off.

Republicans, after all, tend to prize America's role as the world's sole superpower. They see this as crucial to the future of the United States and would not put their own partisan political goals ahead of it. Even if they disagree with Obama's execution of foreign policy, and would say so openly, they refrained from sabotaging him in the way they had on domestic policy. Until the Iran talks.

Republicans are earnestly alarmed about the Obama administration's effort to negotiate a nuclear deal with Iran. They believe Iran is negotiating in bad faith and will exploit any deal to further its nuclear program. Though many analysts find this argument unpersuasive, it is a valid position, and it's fair play to oppose the Iran deal on those grounds. But that opposition has grown into something much bigger than that, and with consequences beyond Iran policy.

Republicans, joined by some Democrats, tried for months to pass new economic sanctions on Iran. The aim was clear: to kill the negotiations, humiliating Obama on the world stage in the process. The US is offering sanctions relief to Iran as part of any deal. By passing new sanctions while the talks are still ongoing, Congress would send the message that the president is not actually in charge of foreign policy and that the US cannot be trusted to uphold its word. Iran would have little choice but to walk away.

Republicans have not been able to pass new sanctions; Democrats, and even a number of Republicans, have seemed unwilling to so openly embarrass their own president on the world stage.

The moment the line was crossed came on January 8, when McConnell and House Speaker John Boehner took matters into their own hands. They secretly arranged for Israeli Prime Minister Benjamin Netanyahu, who also opposes Iran talks and has a famously poor relationship with Obama, to speak to a joint session of Congress urging them to kill the negotiations.

"We are sailing into uncharted waters"

Even Fox News was outraged: here was a foreign ally going behind the president's back, working with an opposition party to undermine the sitting president of the United States. And Republicans were helping him do it.

"We are sailing into uncharted waters," Robert Kagan, a prominent foreign policy hawk who worked in the Reagan-era State Department and later on John McCain's foreign policy team, wrote in an alarmed Washington Post op-ed. "Bringing a foreign leader before Congress to challenge a US president’s policies is unprecedented."

Kagan warned that in some ways, the even greater danger was that such tactics could well become routine: "After next week," he wrote, "it will be just another weapon in our bitter partisan struggle."

After Netanyahu's visit, Republicans went further still. Forty-seven Republican senators signed an open letter, organized by superhawk Sen. Tom Cotton, to Iranian leaders hinting that they could blow up any deal between the US and Iran if they disapproved.

"Congress violates the Constitution by hosting the speech"

The mere act of senators contacting the leaders of a foreign nation to undermine and contradict their own president is an enormous breach of protocol. But this went much further: Republicans are telling Iran, and, by extension the world, that the American president no longer has the power to conduct foreign policy, and that foreign leaders should assume Congress could revoke American pledges at any moment.

"Iran's ayatollahs need to know before agreeing to any nuclear deal," Sen. Cotton told Bloomberg View, that "any unilateral executive agreement is one they accept at their own peril."

A foreign leader reading this letter — whether he or she is Iranian or not — is learning that you are better off walking away than trying to negotiate, in good faith or bad, with the United States of America.

"Between the Netanyahu invite and the Cotton letter, the GOP are blazing new trails in politicization of foreign policy — and debasement of their institutions," David Rothkopf, the CEO of the Foreign Policy group and a scholar at the Carnegie Endowment for International Peace, tweeted. (I've cleaned up the abbreviations common to Twitter.)

In some ways, it looks like Obamacare all over again. Much as Republicans attempted to stop or subvert Obamacare by undermining the institutions responsible for passing and implementing it, they are now seeking to stop or subvert the Iran negotiations by weakening US foreign policy itself. And much as their brinksmanship and obstructionism on Obamacare exacerbated the partisan polarization that has broken Congress, they are now risking similar damage to the ability of the world's lone superpower to conduct its foreign affairs — well beyond just Iran policy.

The polarization of foreign policy puts all US interests at risk, including the security of Israel

There are legitimate policy disagreements over Iran negotiations in Washington, so the line between principled policy opposition and unprincipled partisan sabotage can be blurry. It may help, then, to examine how Republicans' new approach is damaging a US policy that should be less controversial: support for Israel.

The bipartisan consensus on Israel goes back decades. Republican leaders, by inviting Netanyahu to Congress behind Obama's back, and by pressuring members of Congress to side with Netanyahu against their own president, are both exploiting and endangering that bipartisan consensus.

Republicans' hope is that by forcing members of Congress to choose between Israel and Obama, Congress will side with Israel, and thus against Obama. But the risk is that some will side with Obama and against Israel — many Democrats signaled as much by refusing to attend the speech — and that support for Israel will thus become an increasingly partisan issue.

As "pro-Israel" becomes increasingly coded as a Republican issue rather than a bipartisan one, it will likely help Republicans win certain races, but it will substantially erode the consensus on Israel and thus risk eroding US support for Israel. If you earnestly care about Israel, and about the US-Israel relationship, then this trend should alarm you.

Republicans' tactics are so extreme they may be unconstitutional

A premise of Republican meddling on Iran and Israel is that Congress has a right — indeed, a responsibility — for oversight of some aspects of US foreign policy. This is true, and Sen. Cotton's letter points out as an example that the Senate will have to approve any formal treaty between the US and Iran.

Still, Congress' role in foreign policy is constitutionally quite limited. For over 200 years the president has been designated as the "sole organ of the nation in its external relations, and its sole representative with foreign nations," as founding father John Marshall put it in an 1800 speech that the Supreme Court codified into constitutional law in 1936.

The idea of what became known as the "sole organ" doctrine is that the US government needs to be a single unified entity on the world stage in order to conduct effective foreign policy. Letting the president and Congress independently set their own foreign policies would lead to chaos.

There is disagreement over where constitutional law draws the line between what role Congress is and is not allowed in US foreign policy. But it seems awfully clear that House Speaker John Boehner going behind the president's back to negotiate with the Israeli leader violates at the very least the spirit of constitutional limits on Congress.

Indeed, a number of constitutional legal scholars — some of them quite conservative — have questioned the constitutionality of Republicans' actions.

David Bernstein of George Mason University wrote at the Washington Post that Boehner's invitation to Netanyahu "violates constitutional norms that have been observed for generations" and was contrary to the separation of powers. He explained, "Direct diplomatic relations with foreign governments are exclusive in the executive." Boehner disobeyed that.

Another constitutional scholar, Michael Ramsey, put it more simply: "Congress violates the Constitution by hosting the speech." The point is not that John Boehner is going to be dragged before the Supreme Court — he won't — but that Republicans crossed a line that wasn't just a matter of protocol, but of strict and meaningful constitutional limits in how foreign policy is conducted.

And that was before Sen. Cotton and 46 other Republican senators wrote the Iranian leader to tell him to disregard President Obama's promises.

It's worth pointing out there is a law specifically prohibiting US citizens from negotiating with foreign governments without official permission, and thus interfering in the foreign policy of the United States. Called the Logan Act, it's named for a state legislator who corresponded with French officials in 1798 without his government's permission because he disapproved of US policy toward France.

Cotton is not going to face prosecution for violating the Logan Act. "No one is ever actually prosecuted under the measure," legal scholar Peter Spiro wrote recently. "It’s more a focal point for highlighting structural aspects of foreign relations." And that's the point: the letter goes way beyond the legally articulated limits on Congress' role in foreign policy.

The spirit of the Logan Act, like the "sole organ" doctrine, is meant to enforce the idea that the president is in charge of foreign policy. It's not supposed to be like legislation, where Obama and Congress fight it out on a somewhat level playing field. It's meant to be unified. Republicans, by trying to change that, are undermining the very premise of how US foreign policy is supposed to work.

Fracturing foreign policy between the president and Congress would be a disaster for US interests

The greatest harm, should Republicans continue this trend, would be to the ability of the US president to credibly conduct American foreign policy.

Even if you agree with Republicans that Obama's Iran talks are a bad idea, the fact that Republicans have gone beyond opposing a deal to overtly undermining US foreign policy should worry you. Republicans are now freelancing their own foreign policy, conducting shadow diplomacy with both Israel and Iran, dividing US foreign policy against itself.

Who, a foreign leader might reasonably ask, is really in charge in Washington? How can I risk negotiating with the US when Congress might sabotage any deal we strike? How can I make difficult, politically painful concessions to the US if Republicans might end up pulling out the rug from under me? How much can I really trust the US to uphold its word? How safe a bet is working with the Americans?

One of the central lessons of this dysfunctional era in American politics is that one side's overreach quickly becomes the other side's tactic. If you're a Republican, you should ask what you will think if these practices are normalized. What will you think when Democrats in Congress employ these tactics to undermine a Republican administration?

This is not to say that the world will shrug off American leadership; the US is still the Earth's most powerful and important country. But foreign policy is won or lost on the margins more often than you might think. International agreements can succeed or fail with just a smidge more or less trust between the parties. A major US foreign policy challenge this century will be competing for regional influence with powers such as China and Russia. If you're, say, the foreign minister of Myanmar, trying to decide whether to throw in with China or with America, you are going to be a little less likely to hedge toward the US if you think its foreign policy-making apparatus is fundamentally broken.

Throughout Obama's presidency, Republicans have frequently warned that he is projecting insufficient strength or will to maintain America's global standing. It seems odd, then, that their answer to this is to publicly undermine and humiliate the president — and thus sacrifice, for short-term partisan gain, the American resolve and leadership they see as so important for the world.

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