The Senate Republican letter to Iran is, quite explicitly, designed to push the US toward a harder line on Iran. The senator behind the letter, Tom Cotton, has said in previous comments that he wants to blow up the Obama administration's diplomatic approach to the nuclear standoff and instead make a regime change policy.

So it's a more than a little ironic that Cotton's letter could very well end up softening America's Iran policy, or at least undermining his more immediate objective of raising pressure on Iran. Here are three reasons why.

1) The letter could polarize Congress over Iran, making new sanctions harder

There's one way Congress could almost certainly accomplish Cotton's goal of ending US-Iran negotiations: passing new sanctions on Iran. Sanctions relief is Iran's big incentive to agree to limits on its nuclear program; if it's convinced the president can't deliver on that, then the deal is off.

Since Obama has threatened to veto any new sanctions, Republicans would need a fair number of Democratic votes to override him. But Cotton's letter — like the Netanyahu speech invitation before it — could make Iran sanctions, previously a somewhat bipartisan issue, into a more partisan one.

By so aggressively undermining Obama and his presidential authority, the letter is making Iran sanctions synonymous with slapping Obama, and thus polarizing Democrats against a policy they might have otherwise supported.

The New York Times reported furious reactions from Democratic senators to the letter. "I never would have sent a letter to Saddam Hussein," Sen. Debbie Stabenow said, in a voice the Times describes as "shaking with rage." According to Sen. Tim Kaine, "Republicans have made it harder for us to approach this in a careful and bipartisan way."

And Hillary Clinton, the prohibitive favorite for the Democratic presidential nomination, condemned the letter in harsh terms: "Either the senators were trying to be helpful to the Iranians or harmful to the commander in chief."

In Congress, Iran sanctions have becomes perceived as less of a foreign policy issue and more of a partisan political issue. That makes the prospects for a bipartisan coalition in favor of new sanctions potentially a lot less likely, which would make sanctions a lot less likely.

As a result, ironically enough, Obama could have a freer hand to negotiate and perhaps sign a nuclear deal with Iran.

2) The letter could convince Iranians they need to strike a deal as quickly as possible

It's possible that the letter could have its intended effect in Iran: strengthening the hardline position that America can't be trusted and a deal isn't worth the risks. But it's also possible that the letter could have the opposite effect, and will scare Iranian negotiators into getting to a deal now, while Obama is still in office.

This would depend on whether Iranian leaders read the letter as evidence that the US political system is divided to the point that any deal could be revoked and is thus worthwhile, or whether they read the letter as a warning that the next administration won't be as open to negotiations and thus Tehran should take whatever they can now.

By making Obama into the good cop, then, Cotton may actually have made a deal he opposes more likely.

Those in Iran who support cutting a deal with the US could even use this letter to assuage the concerns of hard-liners who say America is never to be trusted. The smart thing, they can say, is to lock America into a deal under Obama. If a future administration wants to break that deal, it will have to breach international law to do so, making it quite difficult to reconstitute the international sanctions regime against Iran.

3) It could lead the world to blame America — and forgive Iran — if talks collapse

Suppose the negotiations with Iran do fall apart. The first American goal — especially for hawks like Cotton — would be maintaining or even ramping up sanctions. The letter might actually make that harder to do.

To really bite, the sanctions need to be international: the US just isn't that important to Iran's economy on its own. In order to persuade European leaders and the UN Security Council to join in new sanctions, the US needs to be able to credibly argue that it was Iran's fault that the deal fell apart. This letter will make that case harder, because it will look like this was all America's fault.

The Iranians may already be planning for this. Iran Foreign Minister Javad Zarif published a response to Cotton's letter that "can be understood as a preemptive attempt to blame a future negotiations collapse on the US," the Atlantic's Jeffrey Goldberg writes. "Advocates of crushing sanctions against Iran might just have undermined their own cause."

This effect could be even more dire if the US and Iran do strike a deal, but then Republicans do exactly as they threaten in their letter and blow up the deal because they don't like its terms. The world would largely blame America for the deal's collapse, and would be hesitant to re-impose the sanctions they had removed as part of the deal.

In this scenario, Iran would get both sanctions relief and would have an excuse to abandon its nuclear commitments under the deal: for Tehran's hard-liners, the best of both worlds.