For at least a decade, the Democrats have been acting like the more mature, reality-based political party.

They have avoided damaging Congressional primaries that give away winnable seats and tear down their own stars. They have not engaged in big, silly conspiracy theories – about, say, skewed polls or global warming. They have been willing to compromise to advance their cause.

The party has much to show for it: a health-care expansion that had eluded the country for decades; L.G.B.T.-rights victories that came faster than once seemed fathomable; the first serious attacks against climate change; a slew of new taxes on the top 1 percent; and the prospect of the most liberal Supreme Court in a generation.

The start of the Democratic convention offers reason to wonder whether the party is in the early stages of a new phase – a self-defeating phase in which a large segment of liberals would rather lose than compromise.

Let’s be clear: The hecklers in Philadelphia are doing Donald J. Trump’s work. They are his allies, no matter how much they may believe otherwise and no matter how honest their passion. Their attempts to inject chaos into the convention will undermine the single best opportunity that Hillary Clinton has to make her case to voters.

...