Otherwise, Mr. Flake said, “this will last decades.”

Most demographers did not believe Arizona could be truly competitive for Democrats in a presidential election until 2020 at the earliest. But Mr. Trump’s unpopularity has spawned a demographic double threat that has implications in Arizona and beyond: He is not just weak among Hispanics, but also with with educated white professionals who have moved to places like Denver, Salt Lake City and Phoenix in search of better jobs and a lower cost of living.

The trouble signs for the November election have been building. In Colorado, the percentage of registered Republicans as a share of the electorate has dropped by four percentage points compared with 2012. Democrats, who now have about the same share of registered voters, carried the state in 2008 and 2012.

In Utah, Mr. Trump’s lack of support with Mormons has allowed Mrs. Clinton to come close to catching him in some polls.

And in Arizona, new voter registration numbers show Democrats have been registering people at a faster rate than Republicans this year. Registered Republicans, however, still outnumber Democrats over all.

“Arizona is on the cusp,” said Ruy Teixeira, a senior fellow with the Center for American Progress, who studies the political implications of population shifts. “And if it is on the cusp this fast, I think that means these other states become even farther out of reach.”

The entire West Coast is already a wasteland for Republicans. The last time one of the coastal states — with the exception of Alaska — went to a Republican nominee was California in 1988. Moreover, losses in Arizona and possibly Utah would leave Republicans safe in just Wyoming, Idaho, Montana and Alaska. The peril for Republicans is evident looking at the Electoral College: Those states only have a combined 13 of the total 538 electoral votes. And even in the likely event that Republicans continue to carry Utah, a win in November would yield only six more electoral votes.