The already unusual campaign for the Republican presidential nomination opened a bizarre new chapter in recent days with Donald Trump’s feelings about the Ku Klux Klan becoming a matter of debate. This followed Marco Rubio’s sudden emergence last week as a far-from-robotic ridiculer of everything from Mr. Trump’s spelling to the source of his tan.

There is a chance that Mr. Rubio’s new needling approach will endear him to voters, but so far the trend lines for the Republican candidates over the past three months haven’t been good: The “would never vote for” percentage for everyone still in the race has been steadily climbing. And all of the Republicans are worse off than Hillary Clinton and Bernie Sanders.

Since last June, YouGov has been surveying the same roughly 3,000 people across America each month. In YouGov’s February survey, 33% were Democrats, 28% were Republicans and 30% were independents, with more than 70% of respondents saying they are registered to vote. In December, January and February, respondents were shown a list of the candidates in both parties and asked which they would or wouldn’t consider voting for. The no-chance percentage has increased for each of the six Republican candidates still in the race. The I’d-never-vote-for-Trump percentage increased to 55% from 51%. The anti-Rubio percentage rose to 39% from 35%. Those who wouldn’t consider voting for Ted Cruz increased to 46% from 41%; John Kasich’s number jumped to 44% from 33% and Ben Carson’s to 47% from 39%.

By contrast, Bernie Sanders’s wouldn’t-vote-for-him tally has stayed at a steady 38% for the past three months. Hillary Clinton has more people writing her off—the no-way response for her stands at 47%, but that is down from 48%. In short, Democrats are having a vigorous campaign that, so far, hasn’t undermined their candidates’ chances in the fall. Meanwhile, Republicans have been destroying each other.

Nasty, protracted primary fights have a history of hurting Republicans for the general election. The 1996, 2008 and 2012 Republican primaries featured months of Republicans attacking each other with ad barrages.

Patrick Buchanan’s scorched-earth campaign in 1996 produced a primary win against Bob Dole in New Hampshire. Mr. Dole went on to win the Republican nomination, but he was damaged by the onslaught. The nomination process dragged out, sapping the Dole campaign’s finances for the general election.

In 2008 and 2012, the primary season featured Republicans targeting each other for months. In both years, the GOP suffered high levels of voter defection in the general election. In 1996 almost 20% of Republican identifiers didn’t vote for Mr. Dole. In 2004, after President Bush’s uncontested primary election, the Republican defection rate was slightly below 5%. In 2008 and 2012 it was over 8%. In those same elections, Democratic defections were about half the Republican rates.

Even among the Republican identifiers in the YouGov survey, there has been serious damage, with every month showing a rising percentage saying they won’t vote for any of the candidates. In general elections, candidates typically lose less than 10% of their base. For now Republican-identifying voters appear unusually inclined not to rally behind the eventual nominee.

According to the February YouGov survey, for Mr. Rubio the loss would be 18%. Mr. Trump would see a 32% decline to vote for him. Mr. Cruz’s loss of GOP support would be 24%, while Mr. Kasich would drop a staggering 33% and Mr. Carson 29%.

The same analysis of Democratic candidates with Democratic identifiers shows that in December 14% said they would not consider voting for Mrs. Clinton and 18% refusing to consider Mr. Sanders. In February these percentages had dropped to 12% and 14%, respectively. Thus, even among each party’s faithful, the numbers are running against the Republicans, which is especially important given that Democrats are a larger share of respondents—33% to 28%—in the YouGov poll.

The general election is usually decided by moderates (34% of the electorate by self classification) with Republicans needing to win a majority of this group to overcome their numerical disadvantage. Two candidates, Messrs. Cruz and Trump, are off the table for moderate voters with 49% and 57% saying they would never vote for them, followed closely by Mr. Carson at 47%.

Mr. Rubio, at 39%, is the only Republican with less than 40% of moderates unwilling to consider voting for him. Democratic candidate Bernie Sanders fares a bit better with 33% of moderates saying they wouldn’t consider voting for him. Crucially, however, Mr. Rubio’s 39% is better than the 42% of moderates who say they won’t consider voting for Mrs. Clinton.

The longer the nominating fight runs, the higher the probability that it adversely affects Republican prospects in November. At this stage, Mr. Rubio is the only Republican in the race who could compete with either Democrat for the moderate-independent vote. Eight months is an eternity in a campaign, and just about anything can happen. But one thing is certain: If the GOP candidates continue to tear each other apart, their Democratic rivals will benefit.

Mr. Brady is a professor of political science at Stanford University and a senior fellow at Stanford’s Hoover Institution. Mr. Rivers is a professor of political science at Stanford, a senior fellow at Hoover and chief scientist at YouGov, an online polling organization.

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