I’ve lived through a Democratic Civil War before. In fact, I’ve been in the middle of two of them. The first was in 1968, when I was the research director for Vice President Hubert Humphrey’s presidential campaign. The second was in 1980, when I was Jimmy Carter’s policy director.

Both times, I watched pressure from the party’s liberal wing tear the party apart and bring down a Democratic presidential candidate. Both times, the Republicans took the White House. Both times, liberal dreams were shattered.

Today, I fear it could all be happening again.

As President Donald Trump moved the Republican Party sharply to the populist right, early entrants to the Democratic Party presidential contest have veered sharply to the left, along with several energetic new Democratic members of the House. The left’s new avant-garde has properly identified the need to confront serious national challenges, from rising income inequality and inadequate health care coverage to climate change.