Republicans are facing a serious loss of support from women voters in the midterm elections, but the Democrats' biggest demographic challenge has been building for more than a decade: they're rapidly losing support from white men without college degrees.

Expand chart Data: Adrian Gray Consulting aggregation of various polls; Chart: Andrew Witherspoon, Harry Stevens/Axios

Why it matters: White voters without college degrees are a shrinking demographic group, but they're still big enough to have a lot of power, even as the overall voting population becomes more racially diverse. The women in this group aren't turning away from the Democrats in big numbers, but the men are — and that could have consequences down the road, if not in November.

Between the lines: A report earlier this year by demographers from four think tanks modeled various scenarios if white non-college voters continue to swing away from the Democrats.

If they shift another 5 percentage points toward Republicans and 5 points away from Democrats, the report found, Republicans would win the White House in 2020 with both the popular vote and the electoral college.

toward Republicans and 5 points away from Democrats, the report found, Republicans would win the White House in 2020 with both the popular vote and the electoral college. In presidential elections after 2020, Republicans would lose the popular vote by bigger margins but still win the electoral college.

Republicans would lose the popular vote by bigger margins but still win the electoral college. If these voters moved back toward the Democrats, they'd win massive presidential victories in both the popular vote and the electoral college.

The bottom line: Other demographic changes could favor the Democrats in the future, but this trend favors Republicans — and will be strong enough to matter for years to come.