Prime Minister Indira Gandhi, in a major shift in political strategy aimed at offsetting her losses to rising regionalism in India, has embarked on a campaign to generate a wave of Hindu nationalism behind her party in the northern states before the next parliamentary elections.

Targeting the vast Gangetic plain that forms the core of Hindu-Aryan India and traditionally fills more than half the seats in Parliament, Gandhi is increasingly appealing to strong religious and linguistic sentiments in what appears to be an effort to arouse opinion against the peripheral, non-Hindi-speaking states that have rebelled against the authority of her ruling Congress (I) Party.

The opposition alliance, concerned that her strategy could keep the Congress Party in power in national elections that may be held as early as next spring--and by January 1985 at the latest--is planning to counter with a campaign accusing the prime minister of indulging in divisive "politics of communalism" in order to gain reelection.

Hindus make up 73 percent of India's population of more than 700 million, compared with the Moslems, 12 percent. But only 20 percent of the population speaks Hindi, the indigenous official language, and many ethnic and cultural differences separate the Hindus of the Gangetic plain from those of other parts of India.

Life in India long has been strained by communal tensions. Although Hindu-Moslem strains have received the most attention, recently there has been an increase in conflict between Hindus and non-Moslem communities, such as the Sikhs, and among the different castes of the Hindu religion.

Paradoxically, the traditional roles of the Congress (I) Party, the champion of national unity, and the Bharatiya Janata Party (BJP)--which has always appealed to northern India's nostalgia for the great Hindu empires--appear headed for a reversal.

"The Congress Party has become the defender of Hindu rights overnight. They're sounding like the RSS," Bharatiya Janata Party leader Atal Behari Vajpayee said in an interview. He was referring to the Rashtriya Swayamsewak Sangh, the extremist organization of Hindu zealots that espouses racial superiority of the heirs of the Aryan conquerers of ancient India.

The BJP, a splinter of the Janata Party that ruled India disastrously from 1977 to 1979, is the successor of the Jana Sangh (People's Forum), the offshoot of the RSS before it merged with the Janata Party in 1977.

Vajpayee, who has led an effort to liberalize the BJP and broaden its constituency to include non-Hindus and extend its influence beyond the northern Hindi-speaking belt, said he is aware of the irony of Gandhi's shift and his own party's transformation.

"But she is not doing it for Hindu rights. She is doing it for herself and her son, to perpetuate her family's power any way she can," said Vajpayee, who was foreign minister in the Janata government. Gandhi's son, Rajiv, 39, is the heir-apparent of the family political dynasty that began with India's first prime minister, Jawaharlal Nehru, her father.

The testing ground for Gandhi's new strategy is the broad swath of Gangetic India known by orthodox Hindus as Aryavarta. It includes the northern states of Uttar Pradesh, Bihar, Madhya Pradesh, Rajasthan, Haryana and the union territory of New Delhi, as well as the predominantly Hindu portions of the states of Punjab and Jammu and Kashmir.

Stung by recent election setbacks in several southern states and sensitive to the growing strength in others, Gandhi appears to have decided to consolidate her strength in the northern Hindi-speaking belt.

Her party has lost more states to opposition parties in the last year than at any time before, except when she was ousted from power in 1977. With a newly formed alliance of south India's regional parties collectively opposing her party's policies, the future of the Congress (I) in that part of the country seems doubtful.

She has shown numerous signs that she plans to appeal to Hindu sentiments. During increasingly frequent visits to the Gangetic plain, she has stepped up her warnings that "divisive forces"--unspecified but implicitly including the rebellious southern states--are bent on "balkanizing" India in the name of regional autonomy. Her opponents say her actions are intended to create an atmosphere in which Hindi-speaking India will rally around her to save the Indian union.

In the Punjab, whose population is nearly equally divided between Hindus and Sikhs, several Hindu rights groups have sprung up, apparently under the sponsorship of Gandhi's party, as forums for militant Hindu nationalism with which the party cannot officially associate.

The groups include the Hindu Unity Movement, whose general secretary is the treasurer of the State Congress (I) Party, and the Society for the Protection of Hindi, which also is reported to have close links to the Congress (I).

During the state assembly election in Jammu and Kashmir in June, Gandhi was reported to have passed up the chance to link Congress (I) with the ruling Moslem National Conference Party so she could be free to pursue the vote in the predominantly Hindu Jammu portion of the state, national conference sources said.

The result was an election campaign tinged with religious communalism and marred by some violence. Congress (I), as expected, lost heavily in Kashmir but made a strong showing in Jammu.

Gandhi has also been criticized by her opponents for failing to anticipate the crisis earlier this year in the far northeastern state of Assam, where immigrants from Bangladesh, mostly Moslems, were attacked by indigenous Assamese, resulting in over 2,500 deaths.

Her critics charge that Gandhi callously forced a state assembly election opposed by the Assamese in an effort to offset Congress (I) losses in south India, while at the same time using the crisis to appeal for unity around her party.

Vajpayee said the Congress (I) Party also has been appealing to the orthodox Hindu vote by floating the rumor that Gandhi has held secret meetings with RSS leaders, creating the impression of some sort of political alliance between the RSS and Congress (I).

"It's designed to attract Hindus, and it's succeeding to some extent," said Vajpayee, adding that a Congress Party ban on RSS members is now largely ignored, and that the government has ceased its once active campaign of weeding out RSS activists from government service.

The danger of creating a wave of strong Hindu nationalist sentiment in northern India--besides creating a schism between Hindu India and non-Hindu India, or Hindi-speaking India and non-Hindi speaking India--is that Gandhi could lose control of the movement and it could grow to challenge the authority of Congress (I), according to Pran Chopra, a political scientist with the Center for Policy Research.

The dilemma faced by the opposition BJP and its National Democratic Alliance partner, the Lok Dal (People's Party), is that by publicly charging Gandhi with playing the politics of communalism and Hindu chauvinism, it will be calling attention to an alleged drift in the Congress (I) that much of the electorate in the predominantly Hindu Gangetic belt would find attractive.

At the same time, the BJP, by Vajpayee's own account, has lost some orthodox Hindu support in its attempts to broaden its base to include non-Hindus and adopt a more moderate position on such issues as the use of English as an official "link" language between Hindi and regional languages.

"We have to expose the tactics of the ruling party," Vajpayee said. "But at the same time we have decided to make the BJP a mass party that will get support from all communities and all regions."