NEW DELHI (Reuters) - India's ruling Congress party has supported calls to ban electoral opinion polls in the world's largest democracy in what the opposition said is an attempt to suppress bad news during a busy election season starting next week.

Congress has fared badly in a raft of recent surveys that show the party is on the back foot ahead of state elections starting next week and a general election due within six months.

"What is the authenticity, what is the scientific process all these agencies are adopting in predicting (results with) these polls?" the Congress party's general secretary Digvijaya Singh said in a television interview on Monday.

Mr Singh said opinion polls were negative because they could influence voters to back candidates seen as winners. He also claimed a polling company has asked him to pay a bribe to secure a favourable result. He did not give details.

The party on Oct 30 backed a proposal by the electoral commission to restrict opinion polls. The commission has long sought to halt such polls once election dates are announced.

They are currently only banned 48 hours before voting begins.

India's size and diversity means that election issues can vary widely between districts and local leaders hold great sway, making results notoriously tricky to predict.

In the run-up to the last national election in 2009, most opinion polls by large agencies correctly forecast the Congress would win more seats than the opposition Bharatiya Janata Party (BJP) but underestimated the size of its winning margin. The Congress-led coalition won 262 seats.

After ten years in power, the government coalition is facing an uphill battle to convince voters it deserves a third term.

Welfare schemes and years of fast economic growth have increased prosperity in much of the country, but polls suggest many voters are favouring the BJP's candidate Narendra Modi, who is running a vigorous campaign blaming the government for high inflation, corruption and a recent economic slowdown.

Several recent polls have put Mr Modi and the BJP ahead in some of the state elections and the general election. The BJP is forecast to win 162 seats in India's 545-seat Parliament next year, versus 102 for Congress, according to a survey by pollsters Team Cvoter for two television networks released last month.

"When the trend of opinion polls is adverse to the political parties, they rubbish them. They start demanding a ban," senior BJP leader Arun Jaitley said in a statement on Monday. "A potential loser in an election cannot seek to alter the rules of free speech."