BERLIN — For the Alternative for Germany, the right-wing populist party that shot to prominence on the strength of its attacks on Chancellor Angela Merkel’s open-door policy toward Middle Eastern and other refugees, the upcoming federal elections in September were supposed to be a moment of triumph.Fresh from the string of electoral successes in state parliaments throughout Germany in the past two years, leaders of the AfD, as the party is known here, until recently hoped to win 15 percent of the votes in the elections, gains that would have made the party one of the strongest voices in the Bundestag behind Ms. Merkel’s Christian Democrats.
