Conor Lamb’s improbable but likely victory in Tuesday’s congressional race in the 18th district of Pennsylvania raises comparisons to the 1974 victories of Democratic candidates running in traditionally Republican districts. The significance of those earlier upsets as precursors to a November wave victory, is recounted in my just-published book, The Class of ’74: Congress After Watergate and the Roots of Partisanship.

In one of those surprise elections, in February 1974, John Murtha, the first Vietnam veteran elected to the House, won a seat in Pennsylvania’s 12th district. Murtha’s military background and moderate views, were at odds with the majority of those Democrats running in November, but they helped him win the votes of district residents who had not sent a member of his party to Congress since 1936.

Even more stunning was the election a few weeks later of Richard Vander Veen in Michigan’s 5th district. The seat was vacant not only because the Republican incumbent had resigned, but because that incumbent was the House minority leader, Gerald Ford, who had been appointed vice president. Ford’s seat had been safely in Republican hands since 1912 and few had expected so secure a seat to change parties just 15 months after Richard Nixon’s landslide victory.

Two more special elections in 1974 — Tom Luken in Ohio and Bob Traxler in Michigan — seemed to set a pattern of Democrats filling vacancies in Republican districts, and panic began to spread among Republicans across the country. The House Judiciary Committee’s impeachments hearings and bipartisan resolutions, the Supreme Court’s decision against Nixon on the Oval Office tapes, the President’s resignation and then Ford’s surprise pardon just two months before Election Day all set the public mood against the GOP. In addition, the steadily worsening economy and growing energy crisis contributed to a decidedly anti-Republican sentiment.

Shortly before the election, Congressional Quarterly predicted “substantial gains” for Democrats, perhaps as many as 35 seats, and teased there was “at least the hint of a landslide.” Independent polls showed voters favoring Democrats in a generic congressional race by 20 points in the Gallup and 24 points in Harris. Democrats led in 18 seats held by Republicans, and were reportedly running even in another 32. Just a few weeks before the election, Ford was playing golf with Majority Leader Tip O’Neill and asked his old friend for a prediction. “It’s going to be an avalanche,” O’Neill advised the startled Ford.

The polls, and the early harbingers of Murtha, Vander Veen and the other special election winners proved prescient. Democrats picked up 49 seats from Republicans, elected 76 new members, and pushed their margin in the House to 291-144. The new members of the Class of ’74, called “The Reinforcements” by New York’s Bella Abzug, joined with veterans to implement sweeping modernization of the House and the Democratic Caucus, disbursing power more widely, imposing discipline on chairmen, and elevating issues of importance from Vietnam withdrawal to energy, the environment and children’s policy.

It may be too early to make predictions about the fortunes of congressional Democrats in November based on Lamb’s successful campaign, but some conclusions are clear. In a wave election, when the public is sufficiently fed up and hungry for change, incumbency and party labels do not provide the level of insulation one had long depended upon. While gerrymandered districts can afford Republicans some protection in typical elections, displeased voters stay home, and many seats thought to be secure can be lost. In 1974, well-entrenched Republicans like New Jersey’s William Widnall and Charles Sandman lost supposedly safe seats.

Another clear lesson is that parties aspiring to the majority must cast their nets wide, not only with voters, but with candidates. Lamb does not unqualifiedly reflect the liberal orthodoxy of the national Democratic Party, although evidence suggests he, like others elected in marginal districts, will reliably vote with the party on most measures once in office. But Lamb is the kind of candidate, running in the kind of district, that Democrats must win to reach the 218 seat majority. As liberal Rep. Phil Burton advised junior members in the mid-1970s, “Get to know southerners and conservatives. Be nice to those guys; listen to them. Without them, there is no Democratic majority.”

Several months ago, Democratic National Committee chairman Tom Perez declared Democrats would not support candidates who were not 100% pro-choice. His remarks drew criticism from many who recalled that, but for congresspeople who were not reliable on key issues, the party would not have won the majority in 2006 or been able to pass landmark legislation like the Affordable Care Act. Democratic activists can have purity or they might just be able to have a majority, but the chances of having both are less than, say, the odds of Conor Lamb being elected to Congress.

John Lawrence, a visiting professor at the University of California Washington Center, worked for 38 years in the House of Representatives, the last 8 as chief of staff to Speaker/Democratic Leader Nancy Pelosi. This post was originally published by John’s blog, Domeocracy.