Sen. Tom Cotton (R-Ark.), who is credited with organizing the letter. (Photo by Justin Sullivan/Getty Images)

Technically, which is to say, legally, you and I are not allowed to travel to Tehran to try to work out a deal between the United States and Iran. There are a lot of reasons why that prohibition makes sense; your ability to gain an audience with the country's leaders being the least of the many difficulties.

But after a doctor named George Logan traveled to France in 1798 to try to prevent war between our two countries, it became illegal for anyone to freelance in that capacity. The law has never been enforced, but it has a habit of popping into the political conversation when someone -- say, most of the Republican conference in the Senate -- tries to influence foreign relations, say, by sending a letter to the Iranian opposition. Which, you're probably aware, has happened.

At the time of Logan's trip, the United States and France were building toward a war, as Michael McConnell, a former federal judge and professor of constitutional law at Stanford University, explained to me by phone. Logan went to France after an official delegation from the United States had been met with demands from anonymous French emissaries for a bribe, an incident known as the XYZ Affair. The Federalist Party of President John Adams was advocating for war, but Logan's visit prompted France to take some actions that defused the situation. Disappointment in Adams's subsequent ramping down of tensions led to the act that bears Logan's name.

As originally drafted, the Logan Act (as it is known) prohibits:

[A]ny unauthorized United States citizen to carry on any verbal or written correspondence or intercourse with any foreign government with an intent to influence its measures or conduct in relation to any disputes or controversies with the United States.

That has been expanded slightly over time, but the intent is the same: Negotiating with foreign governments is the exclusive domain of the executive branch.

But it seems that no one has ever been tried for a violation of the law. "Historically, it has been threatened but never used," McConnell said. "There have been various threats of enforcement over the years, but it's never been enforced." The law has been "frequently violated," he added, "but never enforced."

As written, the law is awfully vague, allowing for a number of possible ways it might be broken. A report from the Congressional Research Service in 2006 recounted the case of the one person who's ever been charged with violating the act: Francis Flournoy, a farmer from Kentucky who wrote a letter in a regional paper in 1803 that advocated a new country west of the Mississippi allied with France. He never faced trial, and the new country -- as you may know -- never came to be.

Nor, McConnell pointed out, are the senators who wrote the letter to Iran likely to be the first punished by the Logan Act. That's because of the constitutional "speech and debate clause," he noted, "which exempts members of Congress from arrest or prosecution for acts done in their official capacity." We noted this last week, when considering the possible charges against Sen. Bob Menendez (D-N.J.). A member of Congress can slander you from the floor of the Senate, and there's nothing you can do about it. "Offhand, I can't think of any reason why this would not fall within that," McConnell said.

There's also an informal policy under which members of Congress defer to the administration on matters of foreign policy, particularly when overseas. That's not anything binding, and, McConnell notes, it too has been somewhat loosely followed over the last 10 to 15 years.

All of which is to say that the White House (and Senate Democrats) are clearly not happy with the senators' letter, but there's not really much that they can do about it. "I would not expect that the attorney general would be bringing any charges," McConnell said drily.

"But who knows," he added. "Stranger things have happened."