India's Narendra Modi will face a key test of his popularity after a series of controversial economic reforms when the state where he forged his political career goes to the polls on Saturday.

Modi built his reputation as an economic reformer in his prosperous home state of Gujarat, which boomed under his rule, attracting investment from around the globe.

But turning around the national economy has proved more difficult and the Modi government reforms have hurt the very constituency of traders and small business owners who were his biggest supporters in the western state.

Analysts say voter anger over the reforms and a desire for change after 22 years of rule by Modi's Hindu nationalist Bharatiya Janata Party (BJP) could boost the flagging fortunes of the opposition Congress Party.

That the Congress campaign has been fronted by Rahul Gandhi -- the man likely to challenge Modi for the premiereship in the next general election in 2019 -- has only added to the pressure on the prime minister.

The election is seen as a chance for Gandhi to finally prove his mettle before he is named president of the party his mother Sonia has led for more than two decades.

"The Gujarat election's national and psychological impact makes it all-important," said Indian political commentator R Jagannathan.

"This state saw Modi's rise, and if he gets humbled here, then even his allies will doubt if he can still win in 2019. An upset here will be unlike any other state."

Modi's personal popularity remains high, with 88 percent of Indians surveyed by the Pew Research Center earlier this year saying they viewed him positively.

Rahul, the 47-year-old scion of the Gandhi-Nehru dynasty that has led the centre-left Congress Party for much of its history, trails behind him on just 58 percent.

But the poll preceded the introduction in July of a new nationwide goods and services tax that has poleaxed small businesses in India, creating widespread anger.

That came just months after a currency ban aimed at tacking widespread tax fraud that created a months-long cash shortage and economists say the two major reforms have hit India's growth.

- Caste politics -

Gujarat has seen major protests, particularly around the city of Surat, home to textile and diamond trading industries that employ tens of thousands of people.

Manoj Agarwal, who heads a local textile traders' association, said they had been hard hit by the chaotic implementation of the new goods and services tax.

"It has hurt all of us, particularly the smaller ones who're being squeezed out under the fresh tax and paperwork burden," he told AFP.

Another threat to Modi's dominance comes from two prominent groups -- the Patidars, who make up almost 14 percent of Gujarat's 43 million voters, and the lowest Dalit caste.

The Patidars, a relatively well-off caste of farmers and traders, came out in force to support Modi in previous polls.

But their 24-year-old firebrand leader Hardik Patel is a fierce opponent of Modi who regularly attracts tens of thousands of supporters to his rallies.

Although at 24 he is a year too young to stand for election in India, Patel has played a key role in the election with his calls for preferential access to government jobs and education for his caste.

He accused the BJP of playing "dirty politics" after a sex tape was leaked online and has entered an informal alliance with Congress.

That alliance also includes Jignesh Mevani, who emerged as a Dalit leader during caste unrest last year sparked by a video of upper caste Hindus publicly beating members of the historically marginalised community that went viral.

Modi has addressed over a dozen rallies, seeking to shore up support ahead of the vote on a whirlwind tour of the state where he grew up as the son of a humble tea-seller.

At a recent BJP rally, thousands of Modi supporters wearing hats in green and orange -- the colours of the BJP -- waved flags and chanted his named.

"Things are not as good as when Modi ji (sir) was here in Gujarat... But he is a leader with a vision," said Rajesh Kumar. "We have to back him."