A scion of India's Nehru-Gandhi dynasty is due to make her electoral debut, potentially strengthening the opposition's challenge to Prime Minister Narendra Modi in parliament after he was humbled in a recent election, politicians and analysts said.

The family has long been the glue holding factions of the main opposition Congress party together, through Modi's massive mandates in 2014 and 2019 and his unexpected loss of a parliamentary majority in elections that ended this month.

Congress has credited Priyanka Gandhi Vadra, 52, its general secretary, and her older brother Rahul, for nearly doubling its tally in the latest election, and galvanising the wider "INDIA" opposition alliance of parties led by Congress, that forced Modi to rely on allies to run a government for the first time.

Rahul and his mother Sonia have both been Congress party presidents and are lawmakers, but Priyanka, known for her oratory and crowd-pulling power, will be standing for office for the first time from one of two seats that Rahul won recently but needs to vacate, in accordance with law. Congress workers have long urged that she contests elections.

"They are getting on the offensive because Priyanka is a good speaker," said political analyst Rasheed Kidwai.

"So in the house, it's Mr. Modi who will be on the defensive because of the presence of the two Gandhis," Rahul and Priyanka, he said. "The Gandhis are on a high now."

'STRONG AND DEDICATED VOICE'

Opposition leader Aaditya Thackeray welcomed Priyanka's candidature: "A very strong and dedicated voice that will only make the voice of INDIA stronger in parliament".

Priyanka will stand for a by-election in the Wayanad constituency in the southern state of Kerala after Rahul, 53, decided to keep his other seat in the family bastion of Raebareli in Uttar Pradesh, India's most populous state that sends most lawmakers to parliament.

Indian laws allow a candidate to contest from up to two seats, but to retain only one.

Analysts expect Priyanka to win from Wayanad, given her brother has won from there twice. Votes are expected to be cast in the coming weeks.

If she wins, she and Rahul will be in the lower house of India's parliament, with their mother Sonia in the upper house.

Modi's Bharatiya Janata Party said Congress's reliance on the family that has given India three prime ministers was not ideal for democracy. BJP leaders, who have long ridiculed Rahul as a "prince" undeserving of his political position, have said Congress has been behaving as if it had won the election.

While Congress has won 99 of 543 seats in the lower chamber of parliament, against 240 for the BJP, the result for Congress is far better than the 52 in 2019 and its all-time low of 44 in 2014.

Priyanka vowed not to "let the people of Wayanad feel the absence of Rahul Gandhi".

"I will work very hard and try my best to please everyone and be a good representative," she told reporters.

(Reporting by Krishn Kaushik; Editing by Bernadette Baum)