Nuclear weapons treaties are like currency exchange rates — always vitally important to the national interest but often stunningly dull, not to say impenetrable. Yet Washington has suddenly been jolted awake by Republican threats to stall if not kill the Obama administration’s New START treaty.

The irony is that by doing so, they would do little to protect American security while providing moral support to Iran, North Korea and any other rogue regime seeking to arm itself with nukes.

By reducing the bilateral limits on deployed warheads and delivery systems, and by modernizing the verification and monitoring system contained in the original START treaty, the new agreement achieved a breakthrough in arms control and improved U.S. relations with Moscow. The equally important strategic objective, however, was to establish a renewed bilateral commitment to arms control that would strengthen the international effort to prevent Iran from building nuclear weapons.

Under Article Six of the Nuclear Non-Proliferation Treaty (NPT), nuclear-armed nations like the United States and the Russian Federation must “pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament.” Although we obviously remain a long way from that latter goal, New START fulfills our obligations to the agreement that represents the sole global obstacle to rogue states building as many nukes as they please. The legitimacy of NPT and the will of the signatory states to enforce its provisions represent our best hope of stemming the spread of dangerous nuclear materials across the planet.

Sometimes regarded with scorn by conservatives in this country, the NPT undergirds the international campaign against an Iranian nuclear weapon — and, for that matter, against the North Korean expansion of its nuclear arsenal. Without it, there is simply no legal bulwark to stop the mullahs and dictators from working their perilous will.

The ratification of New START will enhance our legitimacy and cement our alliances with the Russians and Europeans in the campaign to curtail Iran’s nuclear weapons program. Confronted by this reality, the Iranian regime will no longer be able to claim that its responsibilities under the NPT are somehow excused by the “hypocrisy” of the superpowers, which supposedly should disarm before Tehran responds to world concerns over its nuclear program.

Without New START, the Iranians will once again be tempted to exploit divisions between Washington and Moscow, as they have tried to do so many times before. Indeed, every time relations between the Russians and the U.S. improve, Tehran moderates its position — and every time that central relationship is strained, Tehran is emboldened.

So why would Republicans and conservatives adopt a stance against New START that so plainly benefits Iran — when they are constantly warning about the danger posed by the mullahs and their nuclear ambitions? The most cynical explanation is that, as Senate Minority Leader Mitch McConnell has publicly admitted, their chief concern is to make sure that President Barack Obama serves only a single term. If that is their sole priority, then anything that improves his standing must be stopped or stalled, even if it is in the national security interest of the United States.

But there is an alternative explanation for their stance: pure diplomatic ineptitude. It is worth remembering that the opponents of this treaty are by and large the same geniuses who assured us that we had to invade Iraq to disarm Saddam Hussein, that doing so would tame Iran and that it would cost us nothing. The result was that we wasted trillions of dollars and thousands of lives to discover that Saddam had no weapons of mass destruction — and that our blundering strengthened Tehran immensely.

Richard Burt, the Reagan administration’s chief negotiator for the original START treaty, has noted that “there are only two governments in the world that wouldn’t like to see this treaty ratified — the government in Tehran and the government in North Korea.” To that list may be added the obstructionist leaders of the GOP, whose motives are as questionable as their competence.

Joe Conason writes for The New York Observer.