President Obama’s effort to exploit an international summit for political purposes is dangerous for several reasons. First, whatever he may think of Mr. Trump, politics actually should stop at the water’s edge—otherwise, it will be virtually impossible for the United States to build an effective and sustainable foreign policy without bipartisan support. Even if President Obama has decided to coast through his remaining months in office without attempting to achieve anything important, attacking the likely Republican nominee on an international trip is reckless and disturbing.

Beyond this, however, if Mr. Obama succeeds in establishing his new standard for presidential behavior during election campaigns, he will profoundly undermine both America’s image and his future successors. A raucous year-plus process to fill a four-year job is bad enough inside our borders—does Mr. Obama really want to extend it until it covers the globe? How does he think that new presidents will be able do their jobs after months of nasty attacks from their predecessors? And is this really the way to build a shining city on a hill to inspire a global wave of democratization?

The President’s behavior creates other problems too. Whether or not Mr. Obama is accurately conveying what he has heard when he says that Mr. Trump’s comments have “rattled” foreign leaders, he is using those leaders—in this case, our closest allies—as instruments in a political campaign in a wholly inappropriate effort to imply that they endorse Secretary Clinton. This is damaging not only in importing foreign political preferences into U.S. elections (something that many Americans won’t welcome, especially when Donald Trump has suggested that some of the same allies are not paying their fair share for their own defense) but also in complicating U.S. efforts to work with those very leaders if Mr. Trump is actually elected.