In a strong attack on populism and a defense of the postwar liberal order, Mrs. May called on the U.S. to accept the need for multilateral institutions. She also warned against the “politics of winners and losers, of absolutes and of perpetual strife” — a concept, she said, “that threatens us all.”

The opposition: Mrs. May’s critics believe that some of her own language has fueled an intolerant political climate in Britain. Her inflexibility on Brexit, they say, has left the nation polarized and without a road map out of the E.U.

The markets: Her likely successor, Boris Johnson, says he will keep the option of a no-deal withdrawal from the E.U. on the table. The pound fell to a two-year low over fears that Britain is hurtling toward a chaotic exit from the bloc at the end of October.