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 theWar 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.