Introduction:
After the sudden demise of Lal Bahadur Shastri in 1966, the Indian National Congress had to choose a new leader to take over as the Prime Minister of India. Several senior Congress leaders supported Indira Gandhi for this position due to various reasons.

Key Reasons:

1. Inexperience:
- Indira Gandhi was considered relatively inexperienced in comparison to other senior leaders.
- Some senior Congress leaders believed that she could be easily influenced and controlled, making her a suitable candidate to lead the party.

2. Political calculations:
- Senior Congress leaders saw Indira Gandhi as a politically weak leader who could be easily manipulated.
- They believed that by supporting her, they could maintain their own influence and power within the party.
- They saw an opportunity to have a puppet Prime Minister who would act as per their wishes.

3. Personal loyalty:
- Some senior Congress leaders had a personal loyalty towards Indira Gandhi as she was the daughter of Jawaharlal Nehru, the first Prime Minister of India.
- They believed that she deserved a chance to prove herself and continue her family's legacy in Indian politics.

4. Lack of alternatives:
- The sudden demise of Lal Bahadur Shastri left the party without a clear successor.
- There were no strong alternative candidates who could gain the support of a majority of Congress leaders.
- In such a situation, Indira Gandhi emerged as a compromise candidate who could prevent internal conflicts within the party.

5. Popularity among the masses:
- Indira Gandhi had gained popularity among the masses through her social welfare policies and initiatives.
- Senior Congress leaders recognized the potential of her mass appeal and believed that she could strengthen the party's position in future elections.

Conclusion:
The support of senior Congress leaders for Indira Gandhi as Prime Minister after the death of Lal Bahadur Shastri was driven by a combination of factors such as her perceived inexperience, political calculations, personal loyalty, lack of alternatives, and her popularity among the masses. These factors led to a consensus among senior Congress leaders that she was the most suitable candidate to lead the party at that time.