Current polling shows Trump losing to Hillary Clinton by 6 percent. He's within range in 2012 Obama-carried target states with older populations and many non-college-graduate whites (Florida, Ohio, Iowa and Nevada) but has fallen behind in Pennsylvania and is well-behind in younger-population, higher-education target states (Virginia, Colorado and New Hampshire).
It's possible his standing will improve if he disciplines himself to avoid ad-lib statements that can be spun negatively by the press and lets the continuing dribble of Clinton Foundation and email scandals lead the news. But on Thursday he said, "I'll just keep doing the same thing I'm doing right now."
In which case, many Republicans fear losing their Senate and perhaps even their House majorities unless voters split their tickets more than in recent years. In 2012 only 26 of the 435 congressional districts voted for one party's presidential candidate and the other's candidate for House of Representatives. That's just 6 percent, the lowest percentage since 1920, far behind the 20 percent who split their tickets in 2000 and 25 percent in 1996.
Will more do so in 2016? Evidence that they will comes from current polls showing Republican senators running perceptibly ahead of Trump in target states (Florida, Iowa, New Hampshire, Ohio and Pennsylvania). Similarly, Republican Joe Heck is running slightly ahead of Trump for the Democratic-held open seat in Nevada, Republicans' one possible Senate gain.
Overall, it looks like Republicans have about an even chance to avoid the net loss of four seats that will give Democrats a Senate majority if Clinton wins. So it may make sense for conservative donors to shun Trump and concentrate on these Senate races, in the hope that enough voters will split their tickets.
Ticket-splitting isn't that hard, after all. It has declined in recent years because of the polarization of the electorate and the convergence on issues of each party's presidential and congressional candidates. That started in the 1990s when Bill Clinton's policies cost Democrats congressional seats in anti-gun-control territory but increased his own appeal in upscale Northern suburbs.
It continued in the 2000s as congressional Republicans ran on George W. Bush's record, making small gains in 2002 and 2004 and suffering big losses in 2006 and 2008. It has continued in the Obama years, as the number of moderate congressional blue dog Democrats plunged toward zero.
But Donald Trump is plainly something different and distinct from almost all congressional Republicans, both leadership supporters and tea party rebels. Voters clearly recognize this difference, with non-college whites more favorable to him than Mitt Romney and college graduates less so. Most Senate Republican candidates in close races are incumbents who established their own distinct records long before Trump rode down the Trump Tower escalator and announced his candidacy.
It's not clear whether Republican House incumbents or nominees in open seats have such clear and distinct profiles, but Republican incumbents have clear advantages. They currently hold 247 House seats, the most since the 1920s, and the demographic clustering of Democratic voters enabled Mitt Romney to carry 226 congressional districts, a majority, with just 47 percent of the popular vote. Ace election handicapper Charlie Cook doubts that Democrats have enough strong candidates to make the net gain of 30 seats needed to make Nancy Pelosi speaker again.
In any case, many analysts overestimate the strength of the coattails of landslide winners to sweep ticket mates to victory. That happened in the 1920s and 1930s, but not so much since. Dwight Eisenhower's 1956 landslide didn't install Republican majorities, and Lyndon Johnson's Democratic gains in 1964 owed much to the high job rating of his assassinated predecessor.
Richard Nixon in 1972 and Ronald Reagan in 1984 won landslides but had little in the way of coattails, with about half the districts they carried electing Democratic congressmen. In both years, 44 percent of House districts had split results. John Kerry was one of only three Democrats who lost a George McGovern-carried seat in 1972.
We haven't had a landslide election since 1984, and polling does not point to anything like a 20-point Clinton victory. House Republicans will struggle to deal with Donald Trump's disruptive appeal, but they're likely to keep their majority.