THURSDAY will be a crucial day in the Obama administration’s attempt to stop Iran from acquiring nuclear weapons. No negotiations are scheduled, and the “crippling sanctions” President Obama promised in the absence of diplomatic progress are a ways from approval by the U.N. Security Council. But Feb. 11, the day when Iranians celebrate the 1979 overthrow of the shah, has emerged as another test of strength between the regime of Ayatollah Ali Khamenei and the opposition Green movement. The government has been trying to crush the rebellion with brute force. If it once again fails to stop thousands of protesters from taking to the streets of Tehran and other cities, the West will know that the extremist group that stands behind Iran’s drive for the bomb is one step closer to collapse.

Ever since its manipulation of last June’s presidential election touched off a popular uprising, the extremist clique around Mr. Khamenei and President Mahmoud Ahmadinejad has been entirely preoccupied with the domestic power struggle. Its confusing posturings on the nuclear program — one day appearing to embrace a deal, the next taking a provocative step toward nuclear capability– are calculated with the Green movement foremost in mind. The regime would like to avoid new sanctions that could deepen the popular unrest and so tries, successfully so far, to drive a wedge between Western governments and China. But it also wants to look as if it is defying the outside world — and so in the days before Feb. 11 there have been a stream of missile tests and announcements of new weapons.

Meanwhile, the United States has the opportunity to weaken the regime by doing that which it fears most: providing moral and material aid to the opposition. The administration took a useful step in that direction Monday by issuing a joint statement with the European Union calling on Iran to “end its abuses against its own people” and expressing concern about “the potential for further violence and repression during the coming days.”

More measures should be ready in the event Thursday witnesses an extension of the regime’s crackdown. In addition to condemnations, the United States could join with European allies in applying sanctions to those engaging in repression, including commanders of the Revolutionary Guard. Legislation due to be introduced Thursday by Sens. Joseph I. Lieberman (I-Conn.) and John McCain (R-Ariz.) would require the White House to submit a list of Iranian human rights abusers to Congress; their names would be made public and they would be subject to sanctions, including a freeze on assets and financial transactions.