Sen. Kelly Ayotte (R-N.H.) on Capitol Hill in January. (Alex Brandon/Associated Press)

“No modern precedent exists for the revival of a party so badly defeated, so intensely discredited, and so essentially split as the Republican Party is today.”

Thus wrote George Gilder and Bruce Chapman in “The Party That Lost Its Head,” after Barry Goldwater’s landslide 1964 defeat. The costs of the debacle were very high: The GOP lost 36 House seats, and its Senate contingent was reduced to a corporal’s guard of 32.

No one is likely to bring up the 1964 election when Donald Trump meets Thursday with House Speaker Paul D. Ryan (Wis.) and other leading Washington Republicans. But behind all the talk we’ll hear this week about principles, philosophy and temperament lies a profound fear that Trump’s likely nomination could lead to GOP carnage all the way down the ballot.

At this point, assessing the Trump Effect is speculative because — even assuming, prematurely, a Trump trouncing — not all landslide losses are created equal when it comes to their broader impact. Lyndon Johnson’s win over Goldwater, along with the victories of Ronald Reagan in 1980 and Barack Obama in 2008, had clear down-ticket effects. But the substantial triumphs of Dwight Eisenhower in 1956, Richard Nixon in 1972 and Bill Clinton in 1996 did not call forth comparable congressional earthquakes.

Some Republicans in Congress could be at risk of losing seats, in part because of the party's controversial presidential candidate Donald Trump. These are some the Republicans who are feeling the "Trump effect" the most. (Deirdra O'Regan/The Washington Post)

What is not speculative is the impact Trump’s emergence has already had on vulnerable Republican senators up for reelection this year in blue and purple states. They know Trump is a problem for them, but they confront a dilemma: To win reelection, they need support from middle-of-the-road voters, particularly suburban women, put off by Trump; but they also need help from voters who like Trump. The same will be true in many contested House races.

This tension reduced Ryan, who loves soaring rhetoric about principle, to serving up bureaucratic mush at a news conference Wednesday. When asked about his meeting with Trump, he explained: “We have to go through the actual effort and process of unifying.”

In some cases, the “process” of dealing with Trump has led to some remarkably inartful verbal gymnastics. Sen. Kelly Ayotte (R-N.H.), in a tough race against Democratic Gov. Maggie Hassan, sent reporters scurrying to their thesauruses when her office issued a statement saying she would “support the nominee” but wasn’t “planning to endorse anyone.” Parsing the difference between “support” and “endorse” is likely to endure as a political parlor game.

Republican Sen. Mark Kirk, generally seen here in Illinois as an underdog for reelection against Democratic Rep. Tammy Duckworth, has tried an altogether different and somewhat more straightforward approach. Kirk says he would support Trump, who, he argues, might help Republicans with turnout this fall. But Kirk also goes out of his way to highlight how different he is from Trump on issues ranging from foreign policy to gun control. He’s with Trump except when he isn’t.

In Pennsylvania, Republican Sen. Pat Toomey faces Democrat Katie McGinty in another key race. In an op-ed for the Philadelphia Inquirer, Toomey held back his support for Trump and invited him to earn it. Lest anyone confuse him with Trump, Toomey criticized “his manner and his policies” as well as his “vulgarity, particularly toward women.”

Toomey’s harsh words about Trump would not surprise David Axelrod, the architect of President Obama’s two victories. He believes Trump could have a “really dramatic effect on some of these Senate races.”

In an interview, Axelrod said, contra Kirk, that Trump could suppress Republican turnout and also that many Trump supporters who do vote might skip the rest of the ballot, since “they are not terribly interested in affirming” incumbent officeholders.

On the other hand, in states with large Latino populations — Axelrod cited Arizona, where Republican Sen. John McCain is in a tough contest — Trump could mobilize an unusually large turnout among his Democratic-leaning foes.

And the Chicago political veteran pointed to the paradox that Trump, whose supporters see him as someone who “speaks his mind,” is forcing many Republicans to twist themselves into philosophical pretzels as they try simultaneously to embrace and distance themselves from Trump.

Axelrod spoke of the challenge before Republicans such as Ryan as they try to preserve their standing as principled politicians while also preaching party unity. They will be forced to declare, in effect: “I don’t believe in anything he’s saying, but I support him because he’s the nominee of the party.”

“That’s exactly what people hate about politics,” Axelrod said.

Perhaps especially Trump’s people.

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