“Brexit is clearly a cautionary tale for the Clinton campaign not to get too complacent with a potential victory,” said David B. Cohen, a professor of political science at the University of Akron. “Trump, Sanders and those in Great Britain who ran the Leave campaign are tapping into an anger and anxiety that is clearly festering. Working-class folks in the United States are similar to working-class folks in Europe. And a lot of those working-class people feel as if the international economic system is not working for them and strangling the middle class.”

Mike DuHaime, a Republican strategist, said the British vote was the clearest sign yet that “the intensity against the status quo is far more real than many are still willing to acknowledge.”

“If the Trump victory in the primary wasn’t enough of one, the Brexit vote serves as a major wake-up call indicating just how frustrated average voters are with those in power,” Mr. DuHaime said…

“Liberal internationalism seems to have been dying for a while,” said Mark S. Mellman, a Democratic pollster who is not involved with the Clinton campaign. “But while that may be the animating philosophy of foreign policy intellectuals the world over, it is not the animating philosophy of America, nor of our domestic politics.”…

Frank Luntz, a Republican expert on political messaging, said Mr. Cameron and the “Remain” camp had failed to “personalize, individualize or humanize their campaign.” The “Stronger Together” slogan shared by the “Remain” campaign and Mrs. Clinton feels bloodless and overly intellectual compared with the more emotional, country-first appeals of Mr. Trump and the “Leave” movement, he said.