President Obama’s signing statement in respect of the Trade Facilitation and Enforcement Act will go down in history as a marker of the anti-Israel drift of the Democratic Party. The new law includes a section designed to deter boycotts and sanctions tied to trade with Israel. It puts America squarely against such boycotts and sanctions as a matter of policy and law. The law also makes it clear that it applies to trade with territories Israel controls.

Naturally, President Obama objected. He signed the measure, which passed Congress by a wide margin and with bipartisan support (only one Republican voted against it at the House). But when he inked the measure, he put out a memorably obnoxious signing statement. He derided the law he’d just signed as “conflating” Israel and Israeli-controlled territories and suggested he might defy it.

In fact, the law does not conflate Israel and the territories it controls. This point was marked in the Washington Post by Eugene Kontorovich, a young law professor who has emerged as one of the most brilliant and assiduous defenders of Israel today. He reckons that the “actual effect” of Mr. Obama’s signing statement “is nil.” He notes that Mr. Obama’s statement “does not even purport that parts of the law are unconstitutional or unenforceable.”

To do so would have been an impossible stretch. That’s because of one of the under-appreciated features of the Constitution’s commerce clause. It grants to Congress the power to regulate commerce not only “among the several states” and with “the Indian tribes” but also with foreign nations. It is one of the grants of power that establishes the Congress as the branch of the government that holds the most fundamental of the powers over foreign affairs.

What was so obnoxious in the President’s signing statement was his assertion that “certain provisions” of the law, “by conflating Israel and ‘Israeli-controlled territories,’ are contrary to longstanding bipartisan United States policy.” It’s obnoxious for two reasons. One, the Congress went out of its way not to conflate Israel and the territories it controls. It refers to each of them separately. It’s the President who’s doing the conflation.

The second reason is the reference to certain provisions of the law being “contrary to longstanding bipartisan United States policy, including with regard to the treatment of settlements.” So what? Is the President intending to say that Congress, to which the power to regulate commerce with foreign nations is delegated by the Constitution itself, lacks for the power to change “longstanding bipartisan United States policy”?

Somehow Mr. Obama, in his reference to “longstanding bipartisan policy,” forgot to mention the vote in the Congress against boycotts and sanctions against the territories was a bipartisan tally. The fact is that the only change in policy here is the dogleg away from support for Israel that has been made by Mr. Obama’s administration. This is a tragedy not only for the Jewish people but for the Democratic Party, which has now been marked as the less supportive of Israel, just in time for a new election.