U.S. President Barack Obama pauses while speaking about Iran during his meeting with Secretary of Defense Ash Carter in the Oval Office of the White House in Washington March 3, 2015. (Kevin Lamarque/Reuters)

THE CONCERNS about a prospective nuclear agreement with Iran raised by Israeli Prime Minister Benjamin Netanyahu in a speech to Congress on Tuesday are not — as the White House was quick to point out — new. They had, for example, been spelled out in Senate hearings, as an editorial we published last month recounted. Mr. Netanyahu’s decision to repeat this case before a joint meeting of Congress in defiance of the White House and leading Democrats risked turning what should be a substantive debate into a partisan scrimmage.

Nevertheless, Mr. Netanyahu’s arguments deserve a serious response from the Obama administration — one it has yet to provide. The White House has sought to dismiss the Israeli leader as a politician seeking reelection; has said that he was wrong in his support for the Iraq war and in his opposition to an interim agreement with Iran; and has claimed that he offers no alternative to President Obama’s policy. Such rhetoric will not satisfy those in and out of Congress who share Mr. Netanyahu’s legitimate questions.

His speech singled out “two major concessions” he said would be part of any deal the United States and its partners conclude with Iran. The first is the acceptance of a large Iranian nuclear infrastructure, including thousands of centrifuges for uranium enrichment. The second is a time limit on any restrictions, so that in as little as a decade Iran would be free to expand its production of nuclear materials. Consequently, Mr. Netanyahu said, the deal “doesn’t block Iran’s path to the bomb; it paves Iran’s path to the bomb.”

The Israeli prime minister’s most aggressive argument concerned the nature of the Iranian regime, which he called “a dark and brutal dictatorship” engaged in a “march of conquest, subjugation and terror.” Saying that the regime’s ideology is comparable with that of the Islamic State, he asserted that it could not be expected to change during the decade-long term of an agreement. He proposed that controls on the nuclear program should be maintained “for as long as Iran continues its aggression in the region and in the world.”

In essence, this was an argument that Iran must be sanctioned and contained while its clerical regime remains in power. That has been the explicit or de facto U.S. policy since 1979, but Mr. Obama appears to be betting that detente can better control Iran’s nuclear ambitions and, perhaps, produce better behavior over time. Yet he has shied from explicitly making that case; instead, his aides argue that the only alternative to his approach is war.

Mr. Netanyahu strongly disputed that point. “Iran’s nuclear program can be rolled back well beyond the current proposal by insisting on a better deal and keeping up the pressure on a very vulnerable regime,” he said. Is that wrong? For that matter, is it acceptable to free Iran from sanctions within a decade and allow it unlimited nuclear capacity? Rather than continuing its political attacks on Mr. Netanyahu, the administration ought to explain why the deal it is contemplating is justified — or reconsider it.