NEW DELHI — Rahul Gandhi, the scion of India’s most enduring political dynasty, will take the helm of his family’s party, enshrining his role as chief opponent to Prime Minister Narendra Modi on India’s political stage. Gandhi’s promotion to leader of the Indian National Congress, or Congress party, will officially happen later this month, but his upcoming elevation became clear Monday after no one else filed nomination papers to challenge him. The move was long expected and comes as the party is at a nadir in its fortunes, losing a series of state elections and struggling to counter Modi’s widespread popularity and his upbeat message of development and economic prosperity for all. Gandhi, 47, a three-time member of Parliament, is the descendant of towering figures in Indian history, including his grandmother Indira Gandhi and his great-grandfather Jawaharlal Nehru, who helped lead the country’s independence movement from Britain. Modi, who is campaigning in state elections in his home state of Gujarat against the Congress party, took a dig at the expected victory, comparing the Gandhi dynasty to Muslim Mughal kings of the past. “When Shahjahan came after Jahangir or Aurangzeb came after Shahjahan, were there any elections for that?” he said. “I congratulate the Congress on their ‘Aurangzeb Raj,’ ” he added, comparing Gandhi to the widely unpopular former king. Gandhi, dubbed “the Reluctant Prince” by the Indian media, had long appeared ambivalent about his family’s business, and critics have charged that he is effete and out of touch, despite leading a party that has historically championed India’s poor and found its strength in its villages. In 2014, he led the Congress party’s campaign in a national election in which the party won just 44 seats in the lower house of Parliament, the worst showing in its history. Gandhi briefly disappeared from the scene in 2015 — when he reportedly attended a meditation retreat to contemplate his future — and has appeared re-energized in recent months, analysts say, touring colleges, criticizing Modi with new vigor and hiring new staff members to increase his presence on social media. “People are taking him more seriously