In 1980, Democratic pollster Peter Hart warned Gaylord Nelson, Wisconsin’s champion vote-getter as governor and senator, that he was going to lose. Hart saw a Republican wave coming. Ronald Reagan would defeat President Jimmy Carter and carry other GOP candidates to victory as well.

The opposite of the wave effect in elections is the so-called Eisenhower jacket, a term coined by Democrats predicting that the immensely popular Ike wouldn’t have the coat-tails to help other Republicans down the ballot.

With three months to go in the 2016 race, there is a presumption among most Democrats and more than a few Republicans that Hillary Clinton is headed to a decisive victory. Democrats are talking about a possible wave, while Republicans see a no-coattails election, particularly since Clinton herself remains unpopular.

Of course, the race could change. Trump could get his act together, or there could be a crisis or a Clinton contretemps.

Most wave elections, such as the Democrats’ victories in 2006 and Republican gains in 2010 are in non-presidential years. Exceptions were in 1964, when Lyndon Johnson trounced Barry Goldwater, and in 1980.

An encouraging note for Republicans: The only time in the modern era that an incumbent party ran successfully for a third presidential term, 1988, the makeup of Congress remained virtually the same.

But even a mini-wave could affect the Senate, where 24 of the 34 seats up for election are held by Republicans. The Democrats need a net gain of at least four seats to take control.

It might take a tsunami to capture 30 seats and win the House. All 435 seats may be in play, but with redistricting and population patterns, Democrats have to win the overall vote by seven percentage points to take it back, David Wasserman of the Cook Political Report estimates. The latest NBC/Wall Street Journal poll showed Democrats with a four-point advantage in the general matchup.

Nevertheless, almost daily another House Republican says he or she can’t vote for Trump. Last week Colorado Rep. Mike Coffman ran a commercial declaring, “Honestly I don’t care for him much,” and vowed if Trump is elected: “I’ll stand up to him. Plain and simple.”

Some Republicans say that differentiating their congressional candidates from the nominee may not be hard. “One great thing for many Republican candidates is that Donald Trump is unusual – he’s not a generic Republican,” says Kristen Soltis Anderson, a Republican pollster. “In a state like Ohio, voters will be able to distinguish between Trump and Rob Portman.” Senator Portman is locked in a tight race.

Some endangered Senate incumbents like Mark Kirk of Illinois have flatly declared they won’t vote for Trump. Most others, however, are trying to straddle to avoid offending the fervent Trump followers, while not alienating more independent-minded centrists. New Hampshire Sen. Kelly Ayotte said she’s supporting Trump but isn’t endorsing him. If he comes to her state, she’ll be AWOL.

Some of her supporters want her to distance herself further, especially since Trump gave her an opening last week when he called her “weak.”

Few Republicans are suffering as much as Arizona’s John McCain. Privately he has contempt for Trump. But the senator faces a right-wing primary challenge this month when many Trump supporters will turn out. Thus even when Trump took a shot at him too last week, McCain held his fire and reiterated that he always supports the party nominee.

Several recent polls, however, might widen cracks in this delicate facade. Clinton has opened up huge leads in New Hampshire and Pennsylvania amid signs Trump was dragging down incumbent senators.

Republican strategists say a five-point Clinton victory in these states is survivable for down-ballot candidates. Anything more, particularly if her margin hits double digits, might mean curtains.

Democrats today are more optimistic that these wider margins are possible, noting that in this polarized environment, ticket-splitting is less prevalent. And waves, as Gaylord Nelson discovered in 1980, usually break late.

By Albert R. Hunt

Albert R. Hunt is a Bloomberg View columnist. He was the executive editor of Bloomberg News. -- Ed.

Bloomberg