American presidential elections are largely decided by a diverse and upscale electorate, anchored in America’s cities and suburbs. These communities more closely resemble London than Lincolnshire. Minorities made up more than a quarter of the electorate in the last presidential campaign.

And while Britain decided to leave the European Union through a popular vote, the White House race will be determined by the Electoral College, which is tilted toward the Democrats. Some large states with significant nonwhite populations have been out of reach for Republican candidates for much of the last three decades; California, New York, New Jersey, Illinois and Pennsylvania have voted for every Democratic nominee since 1992. Mr. Obama also won Florida twice, and Mrs. Clinton has a lead there now in part because Mr. Trump is unpopular with Hispanics.

Together those six states offer 166 of the 270 electoral votes needed to win the presidency.

Mr. Trump is at an even greater disadvantage than other recent Republican presidential nominees because of his dismal standing with nonwhite, college-educated and female voters. Unless he can reverse the deeply negative views such voters have of him, he is unlikely to capture the voter-rich communities around Philadelphia, Denver, Miami and Washington that are crucial to winning the White House.

Joe Trippi, a Democratic political strategist who was a consultant for former Prime Minister Tony Blair of Britain, said he expected the Brexit vote to embolden American conservatives. But their excitement, Mr. Trippi said, would be largely “a false read” of the results.

“There are some very similar things — a polarized electorate, nativism, nationalism were clearly big factors, and Trump exemplifies them here,” Mr. Trippi said.

“But there is a difference in the multiculturalism and diversity of the United States, versus nowhere near the same factors in the U.K.”

Despite high levels of concern about immigration and foreign trade, polls show that most Americans have so far recoiled from Mr. Trump’s specific policy proposals, such as deporting 11 million undocumented immigrants.