Republican officeholders resisting presumptive presidential nominee Donald Trump and others still yearning for a conservative alternative share one main goal: ensuring GOP congressional majorities survive the 2016 election, regardless of who wins the White House.

If they want a model, they might look back to 1972 when many top Democrats, opposed to anti-Vietnam War nominee George McGovern, banded together in a post-convention effort to prevent down-ballot damage that November.

My longtime Associated Press colleague Walter Mears called Richard Nixon’s lack of Republican coattails that year his “lonely landslide,” and with good reason.

While Nixon was capturing 49 states and nearly 61 percent of the popular vote, Democrats expanded their Senate majority by two seats to 14 and held GOP House gains to 12, emerging with a still-solid 50-seat majority. That was no accident. Democratic Party leaders and their allies in organized labor did it by converting their last-ditch “Anybody But McGovern” convention effort into a well-funded campaign to help down-ballot candidates.

The key figure was a wily Texan, the late Bob Strauss, who had been the party’s treasurer the previous two years. After the election, he became national chairman and orchestrated the post-election unity that helped Democrats in 1976 band behind former Georgia Gov. Jimmy Carter, a key figure in the 1972 anti-McGovern effort.

A similar split between GOP presidential and congressional campaigns is developing this year. Several top Republican fundraisers said last weekend at the annual Experts and Enthusiasts summit, a festival of ideas hosted by 2012 GOP presidential candidate Mitt Romney, they won’t help Trump. Spencer Zwick, Romney’s finance chair, has signed onto a similar role for House Speaker Paul Ryan, and others indicated they too would focus on congressional races.

Republicans currently have a 247-188 House majority, and most analysts believe only a Democratic tsunami could dislodge it. But their 54-46 Senate majority is shakier, since so many key races are in states Democrats carried in recent presidential elections.

Several surface similarities exist between the Republican situation this year and what Democrats confronted in 1972.

Just as many top Republicans today are resisting Trump’s nomination, many Democrats then saw McGovern as vulnerable to Nixon. Their reasons were different. Democrats feared the impact of the South Dakota senator’s liberal views, especially his outspoken anti-war stance, in that Cold War era, while GOP holdouts question Trump’s temperament and readiness to be president.

Both presidential campaigns were unprepared for the general election, facing divided support from significant party interest groups and prominent elected office-holders, both in Congress and on the state level.

McGovern was narrowly nominated by a disorderly Democratic convention and then had to replace his first vice presidential choice after the disclosure he had electric shock treatment for depression.

Trump won the nomination by relying on the power of his personality, and has been slow to develop a general election infrastructure. Self-funding his primary campaign left him without the means to raise the millions needed for general election advertising and field operations.

But the political landscape is different. Democrats entered 1972 facing an uphill fight to unseat an incumbent president while defending congressional majorities so strong Nixon made them a secondary target.

Republicans in 2016 face a none-too-popular but well-financed non-incumbent, whose party is determined to reduce or replace Republican congressional majorities. While post-2010 reapportionment virtually ensures a continued GOP House majority, a weak Trump showing could threaten its Senate margin because the seven closest races, five with GOP incumbents and two open, are in states that voted Democratic in recent elections.

Many Republican candidates are uncertain how close to get to their prospective presidential nominee. Sen. Mark Kirk of Illinois has rejected him, while others are simultaneously supporting — and criticizing — him.

Even if Trump loses, many Republicans fear he could inflict lasting damage by helping to solidify Democratic leanings among fast-growing groups of minority voters, especially Hispanics and Asian-Americans.

McGovern’s Democratic foes worried his liberal views would create long-term problems, but the Democrats got lucky in the immediate aftermath. Strauss became party chairman and moderated some disruptive internal party reforms. Nixon’s 1974 resignation solidified Democratic congressional majorities and led to Carter’s election in 1976.

But the Democrats continued to suffer from their leftward tilt until Bill Clinton and changing demographics re-established their national presidential majority in the 1990s. That suggests it may take Paul Ryan and his GOP allies time to repair any Trump-inflicted damage.

— Carl P. Leubsdorf is the former Washington bureau chief of the Dallas Morning News.