Lyle Denniston, the National Constitution Center's constitutional literacy adviser, explains the argument over a GOP Senate letter to Iran and its potential violation of an obscure law.

"The letter by 47 Senators and members of Congress trying to undermine negotiations between Iran, the United States [and others] is a pretty clear violation of the Logan Act….Why isn't anyone asking about this and pushing for answers as to how this is not a violation of the law?"

– A comment on Daily Kos, a widely followed website of political and social commentary, on March 10, reacting to the Republican senators' letter on March 9 to the leadership of the Iran government about negotiations over nuclear arms development.

"[Senator] Tom Cotton's letter to the leaders of Iran admonishing them that any agreement entered into today could be reversed by Obama's successor…appears unprecedented for a group of opposition members of Congress to engage in such a communication   It may also be criminal [under the Logan Act]."

– Comment by Peter J. Spiro, a professor of international law at Temple University, on March 9, reacting to that senatorial letter. His remarks were on the website of Opinio Juris, a discussion site focusing on international affairs.

We checked the Constitution, and…

It seems beyond doubt that those who were in on the Founding of the American government did not want ordinary citizens to be trying to act as diplomats in negotiating with foreign nations. In 1799, reacting to the freelancing diplomacy of a Pennsylvania state legislator, George Logan, who had tried to deal directly with the French government at a time of tense relations between the two countries, Congress passed what came to be known (in his dubious honor) as the "Logan Act."

That law – still on the books today, but not even attempted to be enforced since 1803 – makes it a crime with a potential prison sentence of up to three years for "any citizen" of the U.S., acting without government authority, to "directly or indirectly correspond" with any foreign government or foreign official with the aim of influencing their dealings on any controversies with the U.S. government, or of defeating a U.S. government action.

Laid side by side with the "open letter" that 47 Republican members of the U.S. Senate published on Senate stationery on March 9, it would seem that the Logan Act fits. That letter warned the "leaders of the Islamic Republic of Iran" that a deal on the future of nuclear arms development might not be a lasting one unless Congress signed off on it.

First, let's break down what the law says and why it might fit the senators' letter. Then, let's see why the Justice Department might choose, even if the law fits, not to prosecute it. And, finally, let's see what legal defenses the senators might have if, indeed, they were charged with violating the Logan Act.

The letter, of course, was by citizens. One has to be a citizen to get elected to the Senate. It was clearly, in form, a piece of correspondence, called an "open letter." And it hardly had the approval of "the United States," if that is taken to mean the entire national government, or at least the Executive Branch, including the president and the nation's chief diplomat, the secretary of state. The president and his aides in the White House and at the State Department repudiated the lawmakers' overture.

The letter clearly was aimed at a foreign government audience, although it is not clear that it was actually delivered by diplomatic pouch or by the Post Office to Tehran. And no one can have any doubt, realizing how much opposition there is among Republican members of both the Senate and the House to a nuclear arms deal with Iran, that this letter was intended to influence the ongoing talks about such a deal.

Now, in the waning days of Eric Holder's tenure as U.S. attorney general, or in the early days of his chosen successor, Loretta Lynch, should they not go to a grand jury to seek an indictment of the 47 senators? It doesn't take any imagination to suggest that, if one of them did authorize that move, this would ignite a political firestorm, and would further poison White House-Congress dealings on this and on perhaps many other issues.

One of the calculations that Justice Department prosecutors have to take into account is whether there is a real chance that, if a grand jury does issue an indictment, prosecutors could get a jury to convict. That might be a challenge here, because a criminal charge might well look like escalating a political conflict into a serious constitutional dispute over prosecutorial discretion. (Many in GOP ranks already believe President Obama and his administration overuse the powers of the Executive Branch.) An indictment might also rally support to the GOP senators.

But, put aside any cautionary considerations, and imagine going ahead with a grand jury probe, leading to an indictment accusing the senators of violating the Logan Act. What would happen then?

The senators would obviously get good lawyers, who would put on one of more of the following forms of legal defense – either before a case went to trial, or during a trial:

First, argue that the Logan Act was meant to apply only to private citizens, not to senators carrying on public duties.

Second, argue that the senators were engaging in legislative activity – oversight of State Department diplomatic dealings – and that Congress never intended the law to reach that kind of situation.

Third, argue that, since this was legislative activity, the Constitution's Speech or Debate Clause prevents members of Congress from being "questioned in any other place" for what they have done as lawmakers.

The government, perhaps, could counter that the plain language of the law applies to "any citizen," and to "any correspondence" with a foreign government or its leaders, and to any attempt at influencing dealings with the United States.

Further, the government could say that this was not legislative activity: Forty-seven senators are not the Senate, they spoke only for themselves and not for any committee of the full body, and they were making only a political statement that certainly does not qualify for Speech or Debate Clause immunity.

It would make for good political theater, though. And, if this does end up scuttling a nuclear deal with Iran, it would surely make a major political talking point for 2016.

Philadelphia's National Constitution Center is the first and only nonprofit, nonpartisan institution devoted to the most powerful vision of freedom ever expressed: the U.S. Constitution. Constitution Daily, the Center's blog, offers smart commentary and conversation about constitutional issues in the news, drawing insights from America's history and a variety of expert contributors.