It bears emphasizing at the top here that Americans as a whole have become more educated in recent decades, and both parties now have fewer voters with less formal education. But these voters have shifted much more to the GOP and remain a much bigger portion of the Republican pie today, relatively speaking.
According to Pew data, white registered voters with a high school degree or less leaned toward the Democratic Party by a 50-41 margin in 1992, when Bill Clinton was elected to his first term as president. And Democrats still led among this demographic as recently as the late 2000s.
Today, though, they have taken a sharp turn for the GOP, and they currently lean Republican by a whopping 26 points — 59 percent to 33 percent. That's a net shift of 35 points between 1992 and today (see the first graph on the left below).
And it's not just voters whose formal education stopped at high school. Republicans have also gained a bigger advantage among white voters who have attended at least some college but have not graduated (the middle graph). They led by single digits in 1992 and as recently as 2008, but today that lead is 21 points — 57 percent to 38 percent.
A large portion of these shifts, you might notice, began basically as Barack Obama was elected president for the first time.
The GOP's growing advantage among white voters, though, comes to an abrupt halt when you talk about whites with college degrees or more. In fact, Democrats have actually closed the gap among this group (the third graph), drawing even after trailing by as much as double digits in the 1990s and early 2000s.
The shifts are less significant when you look at all voters by education level — not just whites — because Democrats have maintained their dominance among black voters and done increasingly well with Hispanic and Asian American voters.
But even when you include nonwhite voters, those with a high school degree or less have clearly moved toward the Republican Party, while voters with a college degree or more have clearly moved toward the Democratic Party.
As noted above, both parties have technically become more educated over the past two decades. But that's much more the case with the Democratic Party.
Although non-college-educated whites made up 67 percent of Republican-leaning voters in 1992 and now comprise 58 percent, their share of the Democratic pie has dropped over that same span by nearly half, from 59 percent in 1992, to just 32 percent today.
And even as Democrats have lost ground with white voters more broadly, college-educated white voters have increased from 17 percent of Democratic leaners to 25 percent. Over that same span, college-educated whites have remained about a quarter of Republican leaners.
Even if you include non-white voters — and Democrats' rising fortunes with the fast-growing Hispanic population — the portion of the GOP that has less than a college degree has remained much steadier than the Democratic Party. While the share of all GOP-leaning voters without a college degree dropped just 4 points between 1992 and today — from 72 percent to 68 percent — it has dropped by 17 points among Democrats — from 79 percent to 62 percent.
Less formally educated voters have lost clout in American politics in recent years, but they have largely kept their clout in the Republican Party, and the less-educated American voters who remain have moved sharply toward the Republican Party.
Had they not, it's less likely that Donald Trump would be the Republican Party nominee today.