The second problem with Mr. Obama’s strategy is an exaggerated belief in what sanctions can actually achieve. Many American officials seem to think that the embargo on Iranian oil can bring down the entire mullahcracy. Believing that such results are within reach, Washington appears mesmerized by its own sanctions and seems to doubt the utility of using them as a bargaining chip. Consequently, there is growing resistance to lifting them, regardless of what concessions Tehran might offer.

That would be a serious mistake.

We have seen this trap before. Whenever either side believes it has the upper hand, the desire for compromise diminishes, and the desire to checkmate the other side increases. But if we don’t negotiate when we are weak (because we are weak), and we don’t negotiate when we are strong (because we don’t have to), then when do we negotiate?

Moreover, the empirical track record of embargoes does not support the notion that this pressure will bring down the clerical autocracy in Tehran and lead to democracy.

Of the 35 states that have made the transition from authoritarianism to democracy since 1955, only one did so under the weight of an embargo: South Africa. Twenty states endured limited sanctions, and the remaining 14 found the path to democracy without any sanctions. Of the 10 states that endured embargoes, again only one democratized and gave up its nuclear program: South Africa. The others became more undemocratic, reactionary and oftentimes more dangerous.

This is the opposite direction of where we want Iran to go.

If Iran agrees in Moscow to accept the American demand that it halt uranium enrichment at the 20 percent level — too low a level to quickly create a nuclear weapon — this would effectively obstruct any Iranian shortcut to a bomb. Congress must then give Mr. Obama the political space to be able to take yes for an answer.

Congress must make up its mind. Does it want to prevent an Iranian nuclear bomb or does it want to maintain its sanctions? Going forward, it can’t have both.