The assumption should be that a President Clinton would seek to increase taxes on the wealthy, initiate a plethora of new social programs that fall well short of what Bernie Sanders has advocated and stymie the proposed Trans-Pacific Partnership trade accord.

There was some skepticism whether, as candidates, Ronald Reagan and George W. Bush were serious about their huge tax-cut proposals. They were. Or whether Barack Obama really would push to overhaul the health care system. He did.

Sometimes presidents follow campaign commitments that are politically risky. President Bill Clinton fulfilled his promise to ease a ban on gays in the military, which at the time was not popular. And some presidents need to be dragged into breaking an unwise promise. It took the elder George Bush almost two years to abandon his no-new-taxes pledge. The 1990 budget deal, with tax increases, had two effects: It helped usher in the economic good times of the 1990s and cost Mr. Bush politically.

Mr. Trump, who has railed against politicians who don’t keep their promises, could be expected to start deporting 11 million undocumented workers. He said he’d do so over two years without specifying how he would cover the astronomical costs. He would also try to start building the wall along America’s southern border, though it would most likely cost much more than he estimates and he won’t be able to compel Mexico to pay for it.

On trade, international and domestic laws would limit his ability to make good on his vow to impose huge tariffs on Chinese or Mexican goods. He could try to circumvent these restrictions by invoking presidential powers in responding to an “unusual and extraordinary threat.”