Two friends. Two principles. One answer on Iran.

(Yahoo News photo illustration/AP Photos/Graphics Bank)

If Barack Obama has a close friend in Congress, it’s Tim Kaine. Much like Obama, Kaine is a Harvard-trained lawyer who practiced and taught civil rights law before getting into Virginia politics in the mid-’90s. He was an early Obama supporter back in 2007, before it was in vogue, and he served as the new president’s handpicked party chairman before following Obama’s path to the Senate in 2012.

Which is why it’s interesting that Obama and Kaine now find themselves on opposite sides of an important dispute over the president’s Iran deal. Most conflicts in Washington arise from crass partisanship or calculation, making for a kind of dull and predictable theater. But this one has largely played out in private conversation between a couple of legal scholars, and it’s been building for years.

It’s worth trying to step back and understand the broad arguments on both sides here, because there are important principles at stake — not just for how we handle Iran but for our system of government, too.

The deal Obama just struck with the Iranians, in theory anyway, involves the easing of three kinds of interlocking economic sanctions. Some were imposed by executive order of the president, and others by the United Nations. But some of the toughest sanctions were those Congress enacted in 2010, aimed specifically at Iran’s oil imports. (Iran relies heavily on imported oil, even as it sells much of its own supply on the world market.)

As part of the act, Congress gave the president the power to waive some sanctions if Iran took tangible steps away from its pursuit of nuclear weapons. Obama has relied on that provision in recent weeks, saying that his framework with Iran is essentially an exercise of his statutory right to waive the sanctions, and for that reason he can do it alone.

Republicans in the Senate, however, have rallied around something called the “Corker Bill” — named after its main author, Tennessee’s Bob Corker — that would force the president to submit his fragile Iran deal, once the details are worked out this summer, to a rowdy Congress for its approval.

You could pretty easily write this off as more of that predictable partisan thing I was talking about earlier, except that it’s not just Republicans who have expressed support for Corker. It’s also a bunch of Democrats, including one of the bill’s most outspoken sponsors, Tim Kaine.

Obama’s position here is exactly the one you would probably take if you had just spent months haggling your way toward a historic pact with an enemy nation that is, nonetheless, a good deal less hostile and more open to compromise with the White House than most Republicans in Congress are. He has warned that a drawn-out congressional debate might jeopardize the agreement, especially if the Congress that can’t even pass a budget without an existential crisis manages to blow past Corker’s 60-day deadline for action.

But what really gets Obama going, as he has told Kaine and other sponsors, is the idea that the bill will set a new precedent for allowing Congress to meddle in a president’s foreign policy. Historically, while Congress has to ratify full-blown treaties, presidents have had wide latitude to enter into smaller agreements with other governments.

Obama’s fear is that allowing Congress to accept or reject his Iran deal will effectively give Mitch McConnell and his caucus veto power over everything Obama negotiates, including any potential accords on climate change. In other words, that paralyzing, partisan dynamic we all hate about Congress when it comes to fiscal policy or immigration might well be expanded to encompass matters of national security, too. Yay.

This is why Obama says he’ll veto any bill — like Corker’s — that gives Congress the authority to reject the final agreement with Iran. (Corker doesn’t yet have the votes to override that veto, but he’s within field goal range.)

Kaine’s worried about the balance between the branches of government, too, but his fear is the opposite. Even before this Iran debate, Kaine co-authored two pieces of legislation that challenged Obama’s authority to act unilaterally overseas. One would have forced Obama to get a new authorization for war to combat the Islamic State militant group, rather than relying on the authorization Congress gave George W. Bush in 2001; the other would update the War Powers Resolution of 1973, which was supposed to keep presidents from waging extended wars without congressional approval, but which successive presidents have pretty much ignored.

“I’ve had this obsession really since the Iraq War vote in 2002,” Kaine told me when we spoke this week. The problem there, as he saw it, was that Congress was too willing to abdicate its responsibility to act as a check on presidential power, because lawmakers wanted to avoid casting controversial votes.

In this case, Kaine says he doesn’t oppose the Iran deal and would likely vote for it. But since the most critical part of the Iran framework involves rolling back congressional sanctions, he believes Congress has an obligation to scrutinize it.

Kaine has negotiated hard to make the Corker bill more amenable to the White House, and he has argued to the president that a congressional vote is inevitable. “As a practical matter, do you really think you could do a deal and say, ‘This is the way it’s going to be,’ and this Congress isn’t going to get involved?” he asked me. “That’s not realistic.”

I asked Kaine about Obama’s central point — that Republicans might seize on this precedent to stymie anything he tries to achieve in the diplomatic realm from here on out. “That is his concern,” Kaine said. “But he signed up for the same system I did, which is a system with checks and balances.”

Or, to put it another way, you can’t marginalize the democratically elected Congress just because you wish the voters had elected someone else.

So where does all this net out? Both sides have a legitimate case, and when I started looking into it more deeply, I was inclined to side with Obama. Congress gave him the right to waive the sanctions, after all; why should it get to come back now and take that authority away?

But several legal experts, including some who are avid fans of the president, helped persuade me otherwise. There’s no perfect clarity on whether an agreement like this requires congressional approval, and the Iran deal isn’t technically a treaty, like the kind that ends a war.

It’s not so different, though, from arms control agreements reached during the Cold War with the Soviet Union, most of which were brought to Congress for approval. Jack Goldsmith, a conservative Harvard law professor (such things do exist) who writes on the blog Lawfare, has pointed out that, in 2002, the Democratic leader of the Senate Foreign Relations Committee insisted that the Bush administration bring a looming arms reduction agreement with Russia to the Senate as a treaty. That senator was Joe Biden, and Congress did, in fact, ratify the subsequent agreement.

More to the point, Kaine is right that the country hasn’t done very well — nor maintained much unity — with presidents charging ahead on their own or with only ceremonial votes of affirmation, absent full public debates and congressional inquiry. If Obama, the Iraq War critic and constitutional scholar, were still on the other side of Pennsylvania Avenue, it’s hard to imagine that he wouldn’t be standing alongside his friend right now, calling for a binding vote on the Iran deal.

I sympathize with Obama, but it seems to me that he should yield to Kaine on this one, because we’ve learned that it’s better for the country to go through more, rather than less, of a vetting process on an issue central to national security. And then Congress should endeavor to prove to Obama — not to mention the rest of us — that it doesn’t have to make a petty, destructive game of everything it touches.