By driving women, educated white voters and, most significantly, growing blocs of minorities away from the Republican Party, Mr. Trump has hastened social and political changes already well underway in two key regions, the interior West and the upper South, that not long ago tilted to the right.

Now, even as Hillary Clinton contends with inflamed Democratic anxiety over renewed scrutiny of her private email server, these once-red areas — a string of states that voted twice for George W. Bush — are providing an unexpected firewall for her campaign.

Democrats are already strongly confident of victory in three of them — Colorado, Nevada and Virginia — and believe that a fourth, North Carolina, is likely to break their way as well. Added to the party’s daunting advantage in the Electoral College, these states have impeded Mr. Trump’s path to amassing the 270 electoral votes needed to win, limiting his ability to exploit Mrs. Clinton’s late vulnerabilities and forcing him to scrounge for unlikely support in solidly Democratic places like Michigan and New Mexico…

“I think we may be seeing the ground shift under us,” said Senator Tom Udall, Democrat of New Mexico, a member of one of the West’s most enduring political families. “This may be a major political turning point.” Of Mr. Trump, he said, “He’s done some real damage nationally and in the West to Republicans by using some of the nastiest, angriest rhetoric we’ve ever had in politics.”