Politics is always most successfully played as a game of addition, not subtraction. For some reason, however, the Democratic Party establishment thinks the rules have changed.

Supporters reach out to Bernie Sanders at a campaign rally in Cathedral City, California, May 25, 2016. REUTERS/Alex Gallardo

Borrowing a page from GOP front-runner Donald Trump, the Democratic leadership seems determined to belittle Vermont Senator Bernie Sanders, which means belittling his large political following.

Instead of being gracious winners -- establishment favorite former Secretary of State Hillary Clinton is the likely nominee, barring the unpredictable -- Democratic leaders are labeling Sanders a sore loser. They want him to suspend all campaigning or, at the very least, stop criticizing the former first lady. They insist his continuing the race only helps Trump.

I know from insurgency campaigns. I was the campaign manager, as well as chief cook and bottle washer, for African-American Democrat L. Douglas Wilder’s historic 1985 insurgency in Virginia, which pulled down the “No Blacks Need Apply” sign that had been on the door to statewide office in the South.

The Democratic Party establishment belittled us and worked to block Wilder’s unprecedented nomination for lieutenant governor at Virginia’s state convention. It took his antiestablishment independence personally. But the leadership failed to consider the message it sent to Wilder’s legion of supporters. We outfoxed the party leaders, so they were saved from finding out when Wilder won on Election Day.

Yet, the party leaders never learned from this. They worked hard, this time behind the scenes, to block Wilder’s 1989 race for Virginia governor. He won, anyway.

When the 2016 presidential cycle started, the Democratic establishment viewed Sanders as a quixotic candidate. He had long been a thorn in its side -- a “democratic socialist” who accused elites in both major parties of rigging the economic and political systems against working-class families. Democratic leaders were certain the heavily favored Clinton would crush the 74-year-old Brooklyn native.

Instead, he gained traction.

So the establishment adopted a new rosy scenario. It depicted Sanders as the perfect liberal foil to help candidate Clinton build a “centrist” image.

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Now comes the California primary on June 7. Sanders’ detractors are devising yet another rationalization for his reputed political illegitimacy within the Democratic Party.

Instead of denying reality, however, the Democratic establishment should follow President Lyndon B Johnson’s approach in the 1964 presidential campaign.

Polls showed Republicans had virtually conceded the African-American vote when they nominated Senator Barry Goldwater, who had voted against the landmark 1964 Civil Rights Act. As the summer Democratic National Convention approached, LBJ knew he had the political leeway to reject further political demands being made by younger African-American activists.

But LBJ understood political math. He knew that in order for Martin Luther King Jr. to keep his troops united in the fall -- and ready to rally behind the Democratic ticket -- the civil rights leader had to be a winner, not a loser.

So LBJ confided in King what he planned for African-Americans if he won the election. When younger black activists insisted on confronting LBJ, King could then tell them of Johnson’s promised passage of the historic Voting Rights Act and the War on Poverty. With such unprecedented victories within reach, King could not be defied.

Now, of course, Sanders is no King. But LBJ would have understood that the Vermont senator is iconic to his supporters. These voters agree with Sanders’ arguments that the U.S. economic and political systems do not offer a level playing field for all Americans. From my own experience, I know this: Sanders needs to be seen as continuing to fight for principles in order to remain the moral leader his troops demand.

In politics, perception morphs into reality. Sanders and his backers rightly see themselves as winners. They may have lost the nomination because Clinton has won several million more votes. But Sanders supporters believe they hold the moral high ground. The establishment may disagree. But their dedication to a cause isn’t obstructionism: It is a powerful energy the Democratic Party need to harness, not harass.

The leadership apparently assumes it has the whip hand against Sanders -- largely because its advisers point to polls that indicate the Vermonter’s supporters will eventually fall in line behind Clinton to defeat Trump.

LBJ had a far bigger lead over Goldwater than any poll now gives Clinton over Trump. Yet, Johnson still reached out to find common ground with his critics. In 1964, the Democratic Party establishment would never have tried to humiliate Sanders, much less his supporters. It understood the politics of addition.

The Democratic convention in July must have one overriding goal: showcasing unity. True, Sanders is unlikely to be the choice for vice president. But he must remain central to any real Democratic unity.

How can the party do this? History shows that a unity plan has three key parts:

Sanders has spent a lifetime saying he could be trusted to make the right, not the expedient, choice. The Democratic establishment must trust him to do it.

And make it as easy as possible for him to do so.