It's safe to say that no president in modern times has had his legitimacy questioned by the opposition party as much as Barack Obama. But as his term in office enters its final phase, Republicans are embarking on an entirely new enterprise: They have decided that as long as he holds the office of the presidency, it's no longer necessary to respect the office itself.

Is that a bit hyperbolic? Maybe. But this news is nothing short of stunning: A group of 47 Republican senators has written an open letter to Iran's leaders warning them that any nuclear deal they sign with President Barack Obama's administration won't last after Obama leaves office.

It's one thing to criticize the administration's actions, or try to impede them through the legislative process. But to directly communicate with a foreign power in order to undermine ongoing negotiations? That is appalling. And just imagine what those same Republicans would have said if Democratic senators had tried such a thing when George W. Bush was president.

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The only direct precedent I can think of for this occurred in 1968, when as a presidential candidate Richard Nixon secretly communicated with the government of South Vietnam in an attempt to scuttle peace negotiations the Johnson administration was engaged in. It worked: those negotiations failed, and the war dragged on for another seven years. Many people are convinced that what Nixon did was an act of treason; at the very least it was a clear violation of the Logan Act, which prohibits American citizens from communicating with foreign governments to conduct their own foreign policy.

This move by Republicans is not quite at that level. As (Tufts University professor and Brookings Institution senior fellow) Dan Drezner wrote, "I don't think an open letter from members of the legislative branch quite rises to Logan Act violations, but if there's ever a trolling amendment to the Logan Act, this would qualify," and at least it's out in the open. But it makes clear that they believe that when they disagree with an administration policy, they can act as though Barack Obama isn't even the president of the United States.

And it isn't just in foreign affairs. In an op-ed last week in the Lexington Herald-Leader, Mitch McConnell urged states to refuse to comply with proposed rules on greenhouse gas emissions from the Environmental Protection Agency. Never mind that agency regulations like these have the force of law, and the Supreme Court has upheld the EPA's responsibility under the Clean Air Act to regulate carbon emissions - if you don't like the law, just act as though it doesn't apply to you. "I can't recall a majority leader calling on states to disobey the law," said Barbara Boxer, the ranking Democrat on the Senate Environment and Public Works Committee, "and I've been here almost 24 years." The American political system runs according to a whole series of norms, many of which we don't notice until they're violated. For instance, the Speaker of the House can invite a foreign leader to address Congress for the sole purpose of criticizing the administration, and he can even do it without letting the White House know in advance. There's no law against it. But doing so violates a norm not only of simple respect and courtesy, but one that says that the exercise of foreign policy belongs to the administration. Congress can advise, criticize, and legislate to shape it, but if they simply take it upon themselves to make their own foreign policy, they've gone too far.

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But as has happened so many times before, Republicans seem to have concluded that there is one set of rules and norms that apply in ordinary times, and an entirely different set that applies when Barack Obama is the president. You no longer need to show the president even a modicum of respect. You can tell states to ignore the law. You can sabotage delicate negotiations with a hostile foreign power by communicating directly with that power.

I wonder what they'd say if you asked them whether it would be acceptable for Democrats to treat the next Republican president that way. My guess is that the question wouldn't even make sense to them. After all, that person would be a Republican. So how could anyone even think of such a thing?

Waldman is a contributor to The Post's Plum Line blog, and a senior writer at The American Prospect.