India's Congress party and its controlling Nehru-Gandhi dynasty that has been synonymous with political power for most of the 70 years since independence, hit a new low with this weekend's vice-presidential election.

The Congress-backed candidate was crushed in Saturday's parliamentary ballot by the nominee of Prime Minister Narendra Modi and his ruling nationalist Bharatiya Janata Party (BJP).

The victory means the BJP occupy the top three offices of state for the first time, following its victory in last month's presidential ballot.

It heightened a crisis for Congress and raised fresh questions about the party's top leadership -- especially 47-year-old Rahul Gandhi, great grandson of India's first prime minister Jawaharlal Nehru.

Gandhi led the campaign in the disastrous 2014 general election which saw Congress win just 44 seats -- an historic low. The party went on to defeat-after-defeat in state elections.

"The Congress faces two existential crises -- lack of leadership and the absence of an aspirational, coherent vision for the future," Milan Vaishnav, South Asia director at Washington-based Carnegie Endowment for International Peace think-tank, told AFP.

"What is stunning is that the 2014 general election result exposed both of these infirmities, yet the party has made little to no progress remedying them. If current trends continue, the Congress risks terminal decline," Vaishnav added.

While still short of an outright majority, the BJP last week snatched Congress' mantle as the largest party in the upper house, the Rajya Sabha, after almost six decades.

A favourable vice-president could also bolster Modi's legislative agenda as the vice president doubles as chairman of the Rajya Sabha.

- Dynastic crossroads-

The centre-left Congress has ruled India for more than 50 of the past 70 years, most of them with Nehru and his descendants at the helm.

Since Nehru, his daughter Indira Gandhi and grandson Rajiv Gandhi have been prime minister.

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But the so-called 'natural-born leaders' have looked like political outsiders since the 2014 electoral drubbing.

Party number two to his Italian-born mother Sonia Gandhi, 70 -- widow of the assassinated Rajiv -- Rahul has suffered a string of key local election defeats including in the bellwether state of Uttar Pradesh in March.

"Today, unlike the past, the family needs the party more than the party needs the family," R. Jagannathan, a Mumbai-based political commentator and editorial director of Swarajya, told AFP.

"I think that Gandhi name is past its sell-by date. His mother at least had AN interest (in politics), Rahul doesn't seem interested... he is unsuitable for leadership," Jagannathan added.

Rahul Gandhi made an unsuccessful attempt to mediate between regional allies in a dispute that led to Bihar state falling to the BJP last month, and the loyalty he commands within the party is open to doubt.

Jagannathan suggested the once-dominant family should make way for "real grassroot leaders," but Sadanand Dhume of the Washington-based American Enterprise Institute, said a change in leadership would be problematic.

"In theory it makes sense to suggest that Congress should outgrow its reliance on the Nehru-Gandhi dynasty. In reality, the family is all that holds the party together. Take it out and Congress collapses like a proverbial house of cards," Dhume said.

Rahul's younger sister Priyanka, who many see as an alternative leader, is hampered by a controversy over her husband's property dealings, and has so far refused to take a more prominent role.

With the opposition in disarray, the BJP and its allies now rule 18 of India's 29 states, and look set for more gains in the upper house.

"The Rajya Sabha is effectively the only real political check on Modi's power. Once the BJP gains control, Modi will be free to pursue an expansive legislative agenda on a range of issues," Dhume said.

In the current opposition vacuum, even one-time Modi challengers like Bihar chief minister Nitish Kumar now say Modi's victory in the 2019 national election is a foregone conclusion.