Iranian supreme leader Ayatollah Ali Khamenei. (AFP/Getty)

Iranian Supreme Leader Ali Khamenei, the top political authority in Tehran, has sent a public letter to Iranian President Hassan Rouhani praising the interim nuclear agreement that Iran reached in Geneva this weekend with the United States and other world powers. Perhaps just as importantly, the letter also implicitly criticizes Iranian opponents of the deal who'd wanted to get more for Iran. That's especially important not just for Iran's willingness to adhere to this temporary, six-month deal but for its willingness to move toward a permanent agreement in the coming months. Here's the important part, as translated by Al-Monitor's Arash Karami:

What you have gained, an appreciation and thanks for the nuclear negotiation body and officials is befitting, and can be the basis for the next wise steps. Undoubtedly, the ... prayers and the backing of the Iranian nation was the factor in this success, and will be in the future. God willing, persistence against those who want too much has to always be the criteria for the straight path of the officials, and will always be, God willing.

Khamenei has long taken a stance toward the United States that Iranians might call "tough" but Americans would describe as hostile. While he's let the recently elected Rouhani run with his plan to seek a nuclear deal with the U.S. and other world powers, he hasn't taken the lead himself. Ultimately, Khamenei will have to bless any deal in order for it to succeed – and, more than that, he'll probably need to actively help push it past Iranian hard-liners who oppose any compromise with the West.

What makes this letter significant is not just that Khamenei is blessing the deal, but that he's giving Rouhani some political cover in Tehran. This suggests, and is surely meant to broadcast as much, that Khamenei not only supports the deal so far but that he supports it sufficiently that he's willing to publicly pressure Iranian hard-liners to get behind it.

It can be easy for Americans to forget that Iranian politics are complicated and noisy. Khamenei is the ultimate authority but only when he's willing to use that power, which is only true sometimes. With or without his support, getting any deal approved back in Tehran was always going to be one of the toughest parts.

As Kenneth Pollack of the Brookings Institution told me recently, "Khamenei does have, as best as any outsider can tell, all of the power he needs to force this through the system, to overcome the opposition of the hardliners. But. We've never seen Khamenei actually overrule the hard-liners on an issue of this kind of importance." It's possible that Khamenei might decide he is only so willing to publicly confront Iran's hard-liners.

This letter is a positive sign for the deal's prospects to make it through Tehran. It's far from a guarantee, of course – Khamenei could change his mind, could decide he doesn't like the way the deal is going or he could give in to internal pressure. But this is, for now, an important signal he's sending to the world and to his own capital city.