To understand what’s happened, I asked the analysts at the Pew Research Center to dig into their long-running surveys on the country’s mood and break out the numbers just for self-identified Republicans. The numbers generally do not show sharp changes in the last few years. They do show an electorate that has become steadily more radicalized over the last decade.

The disenchantment began during the end of George W. Bush’s presidency, accelerated during the financial crisis and reached a new peak during President Obama’s highly active first term. By some measures, the unhappiness has remained at that peak; by other measures, it’s even higher today.

Consider this: When Pew asks people whether they trust the government in Washington to do the right thing, it offers three possible answers: just about always, most of the time, only some of the time. In the most recent survey, 18 percent of Republicans skipped all of those answers and volunteered “never.” That share was up from 8 percent who did so four years earlier.

Similarly, 33 percent of Republicans said they were angry at the federal government (as opposed to frustrated or basically content), up from 27 percent four years earlier. Only 12 percent of Republicans report being satisfied with the country’s direction — roughly unchanged since 2012 and compared with the 18 percent who gave that answer in October 2008, at the depths of the financial crisis. These patterns suggest that a candidate like Mr. Trump or Mr. Cruz could have emerged in the 2012 cycle, but probably not earlier.

The roots of the anger have two broad causes: one economic, one cultural. The economic cause is the great 21st-century wage slowdown. The typical family makes only marginally more money than it did in the year 2000, because of the financial crisis and disappointing wage growth on either side of the crisis. Historically, nothing breeds political frustration as reliably as economic stagnation.