For days now, the news media has likened the chaotic end of our misadventure in Afghanistan, and the awful images of terrified people scrambling onto planes at the Kabul airport, to the final exit from South Vietnam. The comparison is overdrawn; the last American combat troops left Indochina two years before the collapse of the Saigon government.

But there is at least one potential parallel between the two conflicts that should have President Biden worried: The last time a war blew up in the face of a Democratic president, it derailed his domestic agenda and stalled the most ambitious social reforms of a generation.

To be sure, domestic political concerns should not overshadow the immediate urgency of getting all Americans and the Afghans who worked for them out of Afghanistan. But history shows how adversity abroad has often led to trouble for the governing party back home. Mr. Biden may not be able to save his ambitious legislative agenda unless he understands that lesson from the past.

In 1964, Lyndon Johnson and his fellow Democrats secured crushing majorities that enabled them to enact a flurry of landmark legislation: the Voting Rights Act, the bill establishing Medicare and Medicaid, an overhaul of immigration law. It is a feat Mr. Biden and progressive Democrats in Congress today would dearly like to emulate.

But Johnson’s decision early in 1965 to send thousands of troops to combat the Vietcong soon halted the momentum of his Great Society agenda and put Democrats on the defensive. A year later, as the war dragged on and protests mounted, Johnson’s approval rating dipped below 50 percent. In the midterm contests of 1966, the Republican Party picked up 47 seats in the House, and Democratic governors in eight states were replaced by Republicans — one of them a former actor named Ronald Reagan in California. By 1968, Republicans had taken back the White House, and Democrats never achieved a progressive policy agenda as far-reaching again.

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Two lessons from Johnson’s downfall are paramount. First, tell the truth, even if it makes you look bad, temporarily. The 1971 release of the Pentagon Papers demonstrated that Johnson lied continually when he lauded the progress the United States and its South Vietnamese allies were supposedly making. By 1966, the press was accusing the administration of creating a “credibility gap” that only yawned wider as the conflict escalated.

All presidents lie at times, but those who admit mistakes, particularly obvious ones, can retain their popularity. This happened to John F. Kennedy after the Bay of Pigs fiasco in 1961 and to Bill Clinton when he acknowledged his affair with Monica Lewinsky in 1998 (although he benefited more from the Republicans’ failed attempt to throw him out of office).