Try this thought experiment: You decide to donate money to Mitt Romney. You want change in the Oval Office, so you engage in your democratic right to send a check.

Several days later, President Barack Obama, the most powerful man on the planet, singles you out by name. His campaign brands you a Romney donor, shames you for “betting against America,” and accuses you of having a “less-than-reputable” record. The message from the man who controls the Justice Department (which can indict you), the SEC (which can fine you), and the IRS (which can audit you), is clear: You made a mistake donating that money.

Are you worried?

Richard Nixon’s “enemies list” appalled the country for the simple reason that presidents hold a unique trust. Unlike senators or congressmen, presidents alone represent all Americans. Their powers—to jail, to fine, to bankrupt—are also so vast as to require restraint. Any president who targets a private citizen for his politics is de facto engaged in government intimidation and threats. This is why presidents since Nixon have carefully avoided the practice.

Save Mr. Obama, who acknowledges no rules. This past week, one of his campaign websites posted an item entitled “Behind the curtain: A brief history of Romney’s donors.” In the post, the Obama campaign named and shamed eight private citizens who had donated to his opponent. Describing the givers as all having “less-than-reputable records,” the post went on to make the extraordinary accusations that “quite a few” have also been “on the wrong side of the law” and profiting at “the expense of so many Americans.”

These are people like Paul Schorr and Sam and Jeffrey Fox, investors who the site outed for the crime of having “outsourced” jobs. T. Martin Fiorentino is scored for his work for a firm that forecloses on homes. Louis Bacon (a hedge-fund manager), Kent Burton (a “lobbyist”) and Thomas O’Malley (an energy CEO) stand accused of profiting from oil. Frank VanderSloot, the CEO of a home-products firm, is slimed as a “bitter foe of the gay rights movement.”

These are wealthy individuals, to be sure, but private citizens nonetheless. Not one holds elected office. Not one is a criminal. Not one has the barest fraction of the position or the power of the U.S. leader who is publicly assaulting them.

Associated Press/The News & ObserverBarack Obama at the University of North Carolina, April 24

“We don’t tolerate presidents or people of high power to do these things,” says Theodore Olson, the former U.S. solicitor general. “When you have the power of the presidency—the power of the IRS, the INS, the Justice Department, the DEA, the SEC—what you have effectively done is put these guys’ names up on ‘Wanted’ posters in government offices.” Mr. Olson knows these tactics, having demanded that the 44th president cease publicly targeting Charles and David Koch of Koch Industries, which he represents. He’s been ignored.

The real crime of the men, as the website tacitly acknowledges, is that they have given money to Mr. Romney. This fundraiser of a president has shown an acute appreciation for the power of money to win elections, and a cutthroat approach to intimidating those who might give to his opponents.

He’s targeted insurers, oil firms and Wall Street—letting it be known that those who oppose his policies might face political or legislative retribution. He lectured the Supreme Court for giving companies more free speech and (falsely) accused the Chamber of Commerce of using foreign money to bankroll U.S. elections. The White House even ginned up an executive order (yet to be released) to require companies to list political donations as a condition of bidding for government contracts. Companies could bid but lose out for donating to Republicans. Or they could quit donating to the GOP—Mr. Obama’s real aim.

The White House has couched its attacks in the language of “disclosure” and the argument that corporations should not have the same speech rights as individuals. But now, says Rory Cooper of the Heritage Foundation, “he’s doing the same at the individual level, for anyone who opposes his policies.” Any giver, at any level, risks reprisal from the president of the United States.

It’s getting worse because the money game is not going as Team Obama wants. Super PACs are helping the GOP to level the playing field against Democratic super-spenders. Prominent financial players are backing Mr. Romney. The White House’s new strategy is thus to delegitimize Mr. Romney (by attacking his donors) as it seeks to frighten others out of giving.

The Obama campaign has justified any action on the grounds that it has a right to “hold the eventual Republican nominee accountable,” but this is a dodge. Politics is rough, but a president has obligations that transcend those of a candidate. He swore an oath to protect and defend a Constitution that gives every American the right to partake in democracy, free of fear of government intimidation or disfavored treatment. If Mr. Obama isn’t going to act like a president, he bolsters the argument that he doesn’t deserve to be one.

Write to kim@wsj.com