Supporters of Sen. Bernie Sanders protest outside a Hillary Clinton rally in East Los Angeles on May 5. (Melina Mara/The Washington Post)

Freshman Sen. Ben Sasse (R-Neb.) has been vocally disdainful of his party's presumptive nominee for the presidency. He was contemptous before Donald Trump essentially won the nomination, and afterward he embraced the idea of a third-party candidate to run against both Trump and Democratic front-runner Hillary Clinton. (Some of those pushing a similar idea have proposed that Sasse himself run for president. He says no.)

Over the weekend, at Nebraska's Republican convention, the real challenge of a third-party run became clear. The event, held a few days after Trump won the state by more than 40 points in Tuesday's elections, included a vote on a resolution objecting to a third-party push. As the Omaha World-Herald reported, the resolution was passed by a wide margin.

Why did the state's Republicans reject Sasse's call for a third-party bid? The World-Herald's Robynn Tysver puts it bluntly:

They argued it would only help Democrats win the White House in November. ... “If you support a third-party candidate, you are going to elect Hillary Clinton, and she is going to nominate the next three or four members of the U.S. Supreme Court,” said Pat McPherson, an Omaha Republican.

That's probably true. The challenge for Republicans -- and Democrats -- this year is that most members of each party hate the opposition much more than they love their own candidate. Sasse's plan has a high probability of undercutting Trump's candidacy, likely handing the election to someone whom Republicans like even less.

The Post and ABC News polled on the favorability of Clinton and Trump in March and April. Forty-five percent of registered Democrats had a strongly favorable view of Clinton -- but 75 percent viewed Trump strongly unfavorably. In contrast, 29 percent of Republicans viewed Trump strongly favorably, but more than 80 percent viewed Clinton very negatively.


Over the course of the campaign, it hasn't changed much.


This is why, as Reuters reports, Democrats are already using Trump as a mobilization tool. "Canvassers from the Orange County Democrats carry pictures of Trump when they knock on doors ahead of the June 7 California primary," Luciana Lopez reports. In New Hampshire, the party is distributing lawn signs that tie the likely Republican gubernatorial candidate in that state to Trump's politics.

Cass Sunstein, legal scholar and former White House administration official, has a term for this: "partyism." In 2014, he published a paper suggesting that opposition to members of the opposite party introduced prejudices that, at times, were more powerful than racial motivations. (Hence partyism, as opposed to racism.)

He wrote about his analysis for Bloomberg View that year:

In 1960, 5 percent of Republicans and 4 percent of Democrats said that they would feel "displeased" if their son or daughter married outside their political party. By 2010, those numbers had reached 49 percent and 33 percent. Republicans have been found to like Democrats less than they like people on welfare or gays and lesbians. Democrats dislike Republicans more than they dislike big business.

This shift runs parallel to another shift we've mentioned before: the split between the parties on Capitol Hill. You've seen this graph any number of times, but it's worth revisiting here. Using data from VoteView, we can track the average partisanship of members of each party's congressional delegations. The gap between them is wider than at any point since the Civil War.


(Philip Bump / The Washington Post)

For all of the attention paid to the negative views of Trump and Clinton, that may well be what consolidates the base of each party. Sasse's call to explore another option in presidential balloting is motivated by who Donald Trump is and the policies he espouses. The Republican Party's motivation for voting for him, though, may simply be that he's the best way to prevent the inauguration of Hillary Clinton. And that's an argument that's hard to rebut.