Bernie Sanders is making a big and potentially dangerous mistake with his continuing insistence on changes to the Democratic Party’s rules and platform. I should know. As chairman of Al Gore’s 2000 presidential campaign, I understand too well where such ideological stubbornness can lead.

Back then, many progressives insisted on backing third-party candidate Ralph Nader despite warnings it would undercut the Democratic nominee. Nader received 97,421 votes in Florida, which Gore lost by 537 votes. The result? President George W. Bush, who championed ill-advised tax cuts, the invasion of Iraq and other actions we now deeply regret.

Sanders made an energetic bid for the Democratic nomination, drawing big crowds and fueling debates on important topics such as income disparity. Although he lost to Hillary Clinton, Sanders is pushing his agenda to the party convention and insisting on “reforms” in a Democratic nominating process he describes as seriously defective.

The party should reject Sanders’s demands on grounds of fairness, good policy and smart politics.

Sanders is wrong to suggest the Democratic Party’s nominating system is seriously defective. It isn’t. It’s eminently fair to let party members (i.e., registered Democrats) select the nominee, and to give party loyalists and elected officials (“superdelegates”) a modestly bigger say.

Start with Sanders’ call for open primaries in all states. It might sound little-d democratic on its face, but political parties are not governmental organizations. They are member-run groups that have the right and obligation to set their own rules for picking nominees.

It’s logical and fair to allow only registered or self-identified Democrats to choose their party’s nominee (although numerous states do have open primaries). Letting more non-Democrats choose the nominee doesn’t guarantee success in a November general election. And it does nothing to encourage people to join and work for the party.

Sanders says it’s “really dumb” to refuse to open all primaries to unaffiliated voters. Not surprisingly, he performed better in open primaries than in closed contests. But if a Washington-based Democratic hierarchy can instruct state parties how to run their primaries, let’s just have one big national primary.

Then there’s Sanders’ call to eliminate superdelegates. These are the party leaders — including all Democratic members of Congress, governors and state party chairmen and vice chairmen — who go to nominating conventions unbound by how their states voted, unlike “pledged” delegates. Although he once asked superdelegates to back him, Sanders now paints them as an elite cadre that can thwart the primary voters’ will.

But superdelegates’ enhanced clout is modest. They are careful and mainstream by nature, and they certainly didn’t hand Clinton the nomination, despite the claims of some Sanders supporters. They are party loyalists and workhorses who typically have spent years attending party functions, recruiting candidates and firing up volunteers.

Just look at the tumultuous Republican Party to see how an unpredictable gadfly can hijack a party whose leaders lack resources such as superdelegates.

Key Democratic constituencies, including the Congressional Black Caucus and Congressional Hispanic Caucus, strongly support superdelegates.

“Our delegate selection process is not rigged,” Rep. James E. Clyburn, South Carolina, a senior leader of the Black Caucus, wrote in a letter to colleagues. “It is transparent to the public and open for participation.”

Clinton beat Sanders fair and square. She won more states, more delegates (pledged and super), and 3.7 million more votes than he did.

Nonetheless, Sanders insists the party adopt “the most progressive platform ever passed” at its Philadelphia convention. Since when does the runner-up get to dictate the platform? (Or, for that matter, continue to enjoy Secret Service protection at taxpayers’ expense?) Centrist voters typically decide general elections, so hard-left or hard-right platforms don’t help.

And it’s hard to argue with recent results. Democrats have won the popular vote in five of the last six presidential elections. In 2008, after losing a hard-fought primary to Barack Obama, Clinton promptly endorsed him and campaigned for him. In contrast, Sanders — who refused to even call himself a Democrat until this election — has yet to endorse Clinton. He says she, not he, is responsible for persuading his supporters to back her.

Every vote counts. Sanders should accept the primary outcome and enthusiastically rally his supporters to Clinton’s side to avoid a catastrophic Donald Trump presidency.

William M. Daley is a former commerce secretary and White House chief of staff.