It’s common now for democrats to argue that the agenda they are struggling to implement on Capitol Hill represents the party’s most ambitious since the “Great Society” Congress convened in 1965. That’s a reasonable assessment—but one that the party today should consider as much a warning as an inspiration. Under the relentless prodding of President Lyndon B. Johnson, the Democratic-controlled House and Senate passed landmark legislation at a dizzying pace during that legendary 1965–66 legislative session.

Over those two years, the 89th Congress, finally completing a crusade started by Harry Truman almost two decades earlier, created the massive federal health-care programs of Medicare for the elderly and Medicaid for the poor. It put a capstone on the civil-rights revolution by approving the Voting Rights Act. It created the first large-scale system of federal aid to elementary and secondary schools and launched the Head Start program. It approved breakthrough legislation to combat pollution in the air and water. It created new Cabinet departments, a new agency to regulate automobile safety, and national endowments to fund the arts and humanities. It transformed the face of America with sweeping immigration legislation that finally undid the restrictive quotas that had virtually eliminated new arrivals since the early 1920s.

“It was one of the most productive and impressive Congresses that we’ve had,” says Julian Zelizer, a historian at Princeton University and the author of The Fierce Urgency of Now, a book about Johnson’s push for his Great Society agenda. “Today, it’s unimaginable.”

Then, suddenly, when the work of the 89th Congress was finally finished, Democrats lost 47 seats in the House and three in the Senate during the midterm election of 1966. The Democrats’ bitter disappointment is a cautionary tale for their party descendants hoping to materially improve their odds in next year’s midterm contest by reaching agreement on the sweeping economic bills that have divided the party for months.

The lesson of history is that it is extremely difficult for presidents to translate legislative success in their first year into political success in the midterm elections of their second year. Those early achievements can boost presidents in their reelection bids, but in almost all cases they have not proved an antidote to the other midterm factors that cause the president’s party to lose ground in Congress.