Extraordinary support from those non-college whites is what keeps Trump's national approval from falling below 40 percent. They approve of his job performance by 58 percent to 39 percent while, among all other Americans, 29 percent approve and 66 percent disapprove.
That leaves Republican candidates vulnerable in House districts with above-average numbers of college graduates or non-whites or both. Those include at least half of the 65 seats that David Wasserman of the Cook Political Report has identified as linchpins of the House campaign.
One of them, Ohio's 12th District near Columbus, holds a special election today to replace a nine-term House Republican who retired early. Though GOP nominees carried the seat by double-digit margins in the last two presidential contests, pre-election polls show Democrat Danny O'Connor even with Republican Troy Balderson.
Democratic competitiveness in such a Republican-leaning district reflects superior enthusiasm among Trump's partisan adversaries. Nationally, the House polling average calculated by FiveThirtyEight.com shows voters favoring Democrats over Republicans by roughly 8 percentage points, 47.7 percent to 39.9 percent.
The recent history of lopsided midterm contests suggests that such advantages grow and solidify as the election approaches. In 2006, when Democrats won back the House, and in 2010, when Republicans did, the numbers of majority-party seats rated highly vulnerable by the Cook Report more than doubled between July and November.
The economic fallout of Trump's trade tariffs, which have rattled financial markets and squeezed Trump's farm state allies, represent another source of Republican unease. With government data showing scant wage increases so far for average workers, economic growth and tax-cuts have generated such paltry political momentum that candidates within the reconfigured GOP prefer talking about divisive cultural issues such as immigration and crime.
The ongoing Trump-Russia investigation poses more pre-election danger. Justice Department prosecutors have placed Trump's 2016 campaign chairman Paul Manafort on trial, obtained the cooperation of his former national security adviser Michael Flynn, and seized records from his personal attorney Michael Cohen. What comes next, and when, is not publicly known.