I've been saying for months now that social media is being gamed by the right to divide the left.

The most fervent Bernie Sanders promotions on Reddit are coming from The Federalist or other sites with a preference for Rand Paul. And while Bernie Sanders himself has been quite circumspect and classy about how he speaks of Hillary Clinton, the same cannot be said for some of his self-proclaimed "supporters," who go out of their way to hammer anyone who speaks a positive word about Clinton with derision and scorn.

This is not to say there aren't genuine lefty Bernie supporters out there. There are. But the ones who are genuine seem to manage to behave with a little bit of decorum, unlike others.

It turns out that's entirely intentional.

For months now, America Rising has sent out a steady stream of posts on social media attacking Mrs. Clinton, some of them specifically designed to be spotted, and shared, by liberals. The posts highlight critiques of her connections to Wall Street and the Clinton Foundation and feature images of Democrats like Senator Elizabeth Warren of Massachusetts and Mayor Bill de Blasio of New York, interspersed with cartoon characters and pictures of Kevin Spacey, who plays the villain in “House of Cards.” And as they are read and shared, an anti-Clinton narrative is reinforced. America Rising is not the only conservative group attacking Mrs. Clinton from the left. Another is American Crossroads, the group started by Karl Rove, which has been sending out its own digital content, including one ad using a speech Ms. Warren gave at the New Populism Conference in Washington last May. “Powerful interests have tried to capture Washington and rig the system in their favor,” intones Ms. Warren, as images of Mrs. Clinton with foreign leaders flash by. The new-style digital campaign captures some basic facts about 21st-century communication: Information travels at warp speed on social media, it is sometimes difficult to know where that information comes from, and most people like to read things with which they agree. The result, said Ken Goldstein, a professor of politics at the University of San Francisco who specializes in political advertising, is something more sophisticated. “Politics is usually basic math,” he said, “and this is a little bit of calculus, thinking a couple steps ahead.”

It has worked, too. Environmental activist lion Bill McKibben got punked, much to the right's delight.

A Twitter post recently caught the eye of Bill McKibben, the environmental advocate and godfather of the Keystone XL pipeline protests. It included an image from “The Simpsons” showing Homer and his family basking in mountains of cash in their living room, followed by a report on Hillary Rodham Clinton’s appearing at a fund-raiser with a lobbyist from the Keystone fight. Mr. McKibben’s environmental organization, 350.org, has been trying to raise awareness about the ties it sees between lobbyists for the oil pipeline and former aides to Mrs. Clinton. He promptly shared the post with his 150,000 Twitter followers, and the reaction was immediate. “You expect different from a Clinton?” one person responded on Twitter. And from another: “Did you need another reason not to vote for Hillary Clinton?” Lost in the response was the source of the offending tweet. It was not another environmental organization or even a liberal challenger to Mrs. Clinton. Instead, it was a conservative group called America Rising PAC, which is trying, with laserlike focus, to weaken the woman who almost everyone believes will be the Democratic Party’s candidate for president in 2016.

There are legitimate disagreements between liberals about Bernie Sanders and Hillary Clinton. There are also a lot of Republican SuperPacs with a ton of money willing to game social media to bend impressions -- or solidify them.

I urge everyone to keep their minds open and choose not to be a hater during this primary season. Be for Bernie, be for Hillary, but above all, be for winning.