Since President Obama took office in 2009, his administration has been remarkably tough on Iran, imposing crippling sanctions beyond the Bush administration's, securing global support to contain Iran's economy and repeatedly stating his willingness to use military force against an Iranian nuclear program. But Congress has often found a way to be even tougher.

In late 2009, for example, Congress began pushing legislation that would have punished Iran so severely that administration officials feared it could close off the possibility of diplomacy altogether; State Department officials wrote to Congress warning that the law would be insufficiently flexible and potentially counterproductive. In 2010, 366 members of the House of Representatives sent Obama a letter warning that the U.S.-led sanctions devastating Iran's economy were inadequate and pressing him to do more as soon as possible. And make no mistake: These congressional efforts for tougher action against Iran tend to be deeply bi-partisan.

But that dynamic appeared to change ever so slightly this week. First, 131 House members signed a letter to the administration encouraging it to pursue diplomacy with Iran, which has just elected a reformist new president, Hassan Rouhani. Known as the Dent-Price letter for its principal authors, Reps. Charlie Dent (R-Pa.) and David Price (D-N.C.), it states: "We believe it would be a mistake not to test whether Dr. Rouhani's election represents a real opportunity for progress toward a verifiable, enforceable agreement. ... In order to test this proposition, it will be prudent for the United States to utilize all diplomatic tools to reinvigorate ongoing nuclear talks."

The letter's 131 signers are far from a congressional majority, of course, and include only 17 Republicans. But it's a big deal when a body as traditionally opposed to diplomacy with Iran has so many members publicly calling for it. The National Iranian American Council, a pro-engagement group, called it "unprecedented." Calling for diplomatic engagement with Iran is simply not a policy with a strong domestic constituency.

A second datapoint is sanctions. Usually, every summer, Congress will send a bill to the White House tightening sanctions against Iran, whether the White House thinks that's a good idea or not. But this year, members are breaking tradition: Congressional Quarterly's Emily Cadei reports that Congress's annual sanctions legislation has slipped on the calendar and won't be done until October at the earliest, if it's even ready this year.

As Cadei points out, the delayed sanctions legislation leaves a nice diplomatic window for the United States and Iran: Rouhani takes office in August, and the next round of international nuclear negotiations are planned for September.

Both the Obama administration and Rouhani have long signaled a willingness to engage in direct diplomacy and work toward a wider deal to end decades of animosity. One of the major obstacles is the degree to which Iran and the United States can reconcile their various goals and interests; that's a tough one. These competing interests could only ever be solved by lengthy negotiations and, most likely, difficult concessions from both.

But the other big obstacle to peace is the respective domestic politics of the United States and Iran. Each country distrusts the other's political culture, and both have, in the past, punished politicians who are seen as too weak on adversaries or who got burned in a failed diplomatic outreach. Politics in both countries is also influenced by a significant number of powerful hawks who are axiomatically opposed to any diplomacy. (One of those hawks may be Iranian Supreme Leader Ayatollah Ali Khamenei, whose opposition would be a deal-breaker, although there are some indications he could be more flexible than he seems.)

The point is that U.S.-Iranian diplomacy is, in some ways, as much about the internal debate within each country as about the outward communication between them. For those who would like to see decades of animosity end peacefully and diplomatically, then, Congress's apparent softening on Iran is a potentially significant sign, a real if small step toward peace.