Correction: An earlier version of this editorial incorrectly described Sen. Bob Corker (R-Tenn.) as one of the “announced GOP opponents” of President Obama’s nuclear deal with Iran. Though publicly critical of the deal, Mr. Corker has not yet announced how he will vote on it in the Senate. This version has been corrected.

Sen. Charles Schumer (D-N.Y.) on Aug. 11. (Andrew Burton/Getty Images)

WHEN SEN. Charles E. Schumer (D-N.Y.) decided he would vote against President Obama’s nuclear deal with Iran, he explained his reasoning in a 1,700-word essay. On balance, he concluded, “the very real risk that Iran will not moderate and will, instead, use the agreement to pursue its nefarious goals is too great.” We disagree with that conclusion, but not with serene confidence; we share the senator’s concern that Iran will use the lifting of sanctions to intensify its toxic behavior in the region. We understand and respect Mr. Schumer’s decision; also, it’s generally better to treat policy disagreements in good faith.

That has not been the spirit in which Mr. Obama and his team have met his Iran-deal critics. The president has countered them with certitude and ad hominem attacks, the combined import of which is that there are no alternatives to his policy, that support for the deal is an obvious call and that nearly anyone who suggests otherwise is motivated by politics or ideology. Mr. Obama’s rhetoric reached its low point when he observed that the deal’s opponents value war over diplomacy and that Iranian extremists were “making common cause with the Republican caucus.”

This was self-contradictory when the president said it; one GOP critic, Senate Foreign Relations Committee Chairman Bob Corker (Tenn.), is a man Mr. Obama himself praised, just four months ago, as “sincerely concerned about this issue” and “a good and decent man.” The White House lost further consistency after Mr. Schumer’s announcement. If there’s anyone who’s not a Republican partisan, it’s the arch-Democrat from New York, who’s planning a bid to lead the Democratic Senate caucus after the current leader, Harry Reid (Nev.), retires. As payback, the White House and its allies are openly encouraging Democrats to deny him the job.

After six-plus years of a presidency in which Mr. Obama has himself been the target of relentless, often unfair, often purely partisan attacks, we can understand why he’s gotten a bit jaded about seeking bipartisan support and feels justified to respond in kind. Given his need to retain enough Democratic votes to sustain a veto if both chambers reject the Iran deal, we also get the raw political necessity of making an example out of Mr. Schumer.

Still, by not sticking to the merits of the deal, Mr. Obama implies a lack of confidence in them. The contrast is striking between the president’s tone today and his 2008 speech accepting the Democratic nomination: Looking ahead to debating his GOP opponent, Sen. John McCain (Ariz.), he pledged that “what I will not do is suggest that the senator takes his positions for political purposes, because one of the things that we have to change in our politics is the idea that people cannot disagree without challenging each other’s character and each other’s patriotism.” There’s a sad progression from that aspiration to an approach that is all about winning, even if it has to be winning ugly.