It was predictable that this would not last. Polls consistently show that Americans do not want gridlock; they want Washington to work. And plenty of the people coming to Mr. Paul’s rallies were from the “Keep your government hands off my Medicare” wing of the Tea Party, who were not going to like his approach to spending reductions once it was clear that meant cutting seniors’ entitlements.

By his second year as a senator, Mr. Paul had shattered the illusions of a good number of his supporters.

During the government shutdown of 2013, he was caught on a live microphone advising Senator Mitch McConnell of Kentucky, then the leader of the Republican minority, that the party could win the messaging war if it highlighted its willingness to compromise.

Mr. Paul reversed his pledges on issues like cutting military spending and foreign aid.

He was discussed as a presidential prospect almost as soon as he won his seat. But with each move toward the mainstream, he lost some of the fervid supporters who had powered that win.

In 2010, Mr. Paul ran against the McConnell machine in Kentucky. His opponent in the Republican primary was Trey Grayson, the secretary of state and a McConnell protégé who had the right résumé (Harvard, top of his law school class at the University of Kentucky) but unfortunate timing.

I followed the Paul campaign as I covered the Tea Party for this newspaper and wrote about it in the penultimate chapter of my book, “Boiling Mad: Inside Tea Party America.” I began hearing about Mr. Paul from Tea Party activists in January, when his candidacy seemed like libertarian fantasy. By the time I spent four days trailing him in April, it was pretty clear that he was going to win. When I returned to see him win the primary in May, his supporters were on Twitter declaring his margin the #randslide. (In between, there were persistent updates from his campaign manager, a local blogger and true-believer libertarian who was replaced soon after the primary win.)