President Obama warned Iran’s ayatollahs on Tuesday that he was ready to walk away from a nuclear deal if Tehran won’t agree to tough measures to prevent cheating.

Let’s hope he means what he says.

Negotiators have already extended the deadline from June 30 to July 7. The administration should push it back further if Tehran won’t give inspectors adequate access or clear up questions about previous military dimensions of Iran’s nuclear program.

“Given past behavior on the part of Iran,” said the president, verification can’t “be a few inspectors wandering around every once in a while.” Rather, he said, there’s “going to have to be a serious, rigorous verification mechanism. That, I think, is going to be the test as to whether we get a deal or not.”

So far Iran has been unwilling to meet that test.

Ayatollah Ali Khamenei, Iran’s supreme leader, recently ruled out inspection of Iranian military sites, where much of the nuclear work has been conducted, or interviews with nuclear scientists. The ayatollah’s red lines walk back provisions that negotiators thought were already agreed to. In the meantime, Yukiya Amano, head of the United Nations nuclear watchdog, the International Atomic Energy Agency, is meeting this week with top Iranian officials in Tehran, who are still stonewalling on questions the IAEA submitted in 2013.

If those questions aren’t answered and if Iran won’t agree to tough, enforceable verification, then U.S. negotiators should play hardball. A further deadline delay (or even a break in talks) doesn’t mean negotiations are over. By demonstrating that the president means what he says about adequate verification, a break in talks may offer the only hope of an acceptable deal.

For one thing, it may convince the ayatollahs that the president has some red lines he will not abandon. Until now, the major concessions have come from the United States and its P5+1 negotiating partners (the five permanent members of the U.N. Security Council plus Germany). The P5+1 abandoned early efforts to limit Iran’s nuclear activities to what was required for a peaceful energy program. Instead, Tehran has been permitted to maintain its current program, equipment, and facilities, under temporary limits, restrictions, and safeguards - many of which will expire in 10 to 15 years.

This deal would indeed inhibit Tehran from acquiring means and materiel for a bomb for at least a decade and maybe longer, and would make it difficult for the Iranians to “sneak out” or “break out” in less than a year. But given Iran’s history of secret programs, the deal is worthless without tight inspection measures. And it must also contain clear provisions to reinstate sanctions if Iran cheats - provisions that can’t be vetoed by Russia, China, or Iran.

So far the ayatollahs seem to believe that Obama will cave on verification because he wants the deal so intensely. But a bad deal would tarnish Obama’s legacy, and the president appears to get it. He also knows the U.S. public isn’t clamoring for an agreement, about which Congress is leery.

Indeed, it is Tehran that is badly in need of a deal.

Iran is “hemorrhaging hundreds of billions because of sanctions and tens of billions because of the drop in oil prices,” points out the Carnegie Endowment’s Karim Sadjadpour, one of the shrewdest U.S. experts on Iran. “If this deal is signed or isn’t signed, I think it’s not going to really affect the finances of most Americans, whereas in Iran the entire nation is waiting on the edge of its seat for this deal to happen.”

In other words, the deal is an economic necessity for the Iranian people, but a conundrum for Khamenei, whose ideology calls for resisting the Great Satan. Khamenei is trying “to reconcile the economic imperatives of the nation with the political imperatives of the hard-line elite,” says Sadjadpour, who has written extensively on the supreme leader.

If Obama sticks to his red lines, Khamenei will have to choose.

Even if the Iranian leader balks, the negotiations game isn’t over. The ayatollah may believe the P5+1 will blame the United States if the talks halt and will break ranks on sanctions. But this isn’t likely to be the case.

“France, the U.K., Germany, and even Russia and China have seen that the Obama administration and Secretary (John) Kerry have really been committed to trying to resolve this issue diplomatically,” says Sadjadpour. So the P5+1 is likely to stick together and the sanctions are likely to hold.

Nor is Iran likely to rush to ramp up its nuclear program if the talks break down. “I don’t see them putting their foot on the gas,” says Sadjadpour, “because if they do, that is going to trigger additional sanctions from the United States. And then they will lose the blame game, right?”

If there is no accord by the new deadline, the more likely result will be a stalemate that disappoints the Iranian people and forces Khamenei to reconsider. But whether or not that is the case, something has to give in Tehran before the talks can be concluded. As Obama made clear - and hopefully he meant it - without rigorous verification there cannot be a deal.

Trudy Rubin is a columnist with the Philadelphia Inquirer.