One reason to worry that Mr. Trump may try to seize the moment by championing the protesters is that it has become an article of faith among President Barack Obama’s critics than in 2009 he missed a golden opportunity to do just that, when many Iranians took to the streets after a disputed election result. But it was never clear what difference American rhetorical support would have made then, other than allowing the Iranian government to depict the protesters as American lackeys, giving the security services more of a pretext to crack down violently.

Even if Mr. Obama’s support might have somehow been helpful to the Iranian opposition, Mr. Trump’s almost certainly will not be. Whatever Iranians think of their own government, they are unlikely to want as a voice for their grievances an American president who has relentlessly opposed economic relief for their country and banned them from traveling to the United States.

In addition, Mr. Trump is now threatening to “terminate” the nuclear deal (breaking with European allies and the rest of the United Nations Security Council); unconditionally supports Iran’s biggest adversaries, Saudi Arabia and Israel; and recently recognized Jerusalem as Israel’s capital, a move rejected by every other country in the region. His policies are dividing the United States from its international partners and giving Iranians reasons to unite against him. A smarter strategy would be designed to do the opposite.